boards/arm/stm32/nucleo: Fix BBSRAM compilation

[jlaitine]

Summary

Fix the compilation of stm32f4/7 nucleo boards with:
CONFIG_STM32F7_BKPSRAM=y
CONFIG_STM32F7_BBSRAM=y
CONFIG_STM32F7_PWR=y
CONFIG_STM32F7_SAVE_CRASHDUMP=y

Impact

This fixes regression that the BBSRAM code doesn't compile any more. There are no configurations in CI enabling this, so the breakage has gone unnoticed.

Testing

Tested with enabling the crash dump saving on nucleo-f767zi:nsh, boards/arm/stm32f7/nucleo-f767zi/configs/nsh/defconfig.

Building without the fix results:

./tools/configure.sh nucleo-f767zi:nsh
make

CC:  board/stm32_bbsram.c board/stm32_bbsram.c: In function 'board_crashdump':
board/stm32_bbsram.c:429:25: error: 'stack_t' has no member named 'interrupt'
  429 |       pdump->info.stacks.interrupt.sp = sp;
      |                         ^
board/stm32_bbsram.c:434:25: error: 'stack_t' has no member named 'user'
  434 |       pdump->info.stacks.user.sp = pdump->info.regs[REG_R13];
      |                         ^
board/stm32_bbsram.c:441:25: error: 'stack_t' has no member named 'user'
  441 |       pdump->info.stacks.user.sp = sp;
      |                         ^
board/stm32_bbsram.c:444:21: error: 'stack_t' has no member named 'user'
  444 |   pdump->info.stacks.user.top = (uint32_t)tcb->stack_base_ptr +
      |                     ^
board/stm32_bbsram.c:446:21: error: 'stack_t' has no member named 'user'
  446 |   pdump->info.stacks.user.size = (uint32_t)tcb->adj_stack_size;
      |                     ^
board/stm32_bbsram.c:482:61: error: 'stack_t' has no member named 'user'
  482 |       stack_word_t *ps = (stack_word_t *) pdump->info.stacks.user.sp;
      |                                                             ^
board/stm32_bbsram.c:489:27: error: 'stack_t' has no member named 'user'
  489 |   if (!(pdump->info.stacks.user.sp <= pdump->info.stacks.user.top &&
      |                           ^
board/stm32_bbsram.c:489:57: error: 'stack_t' has no member named 'user'
  489 |   if (!(pdump->info.stacks.user.sp <= pdump->info.stacks.user.top &&
      |                                                         ^
board/stm32_bbsram.c:490:27: error: 'stack_t' has no member named 'user'
  490 |         pdump->info.stacks.user.sp > pdump->info.stacks.user.top -
      |                           ^
board/stm32_bbsram.c:490:56: error: 'stack_t' has no member named 'user'
  490 |         pdump->info.stacks.user.sp > pdump->info.stacks.user.top -
      |                                                        ^
board/stm32_bbsram.c:491:29: error: 'stack_t' has no member named 'user'
  491 |           pdump->info.stacks.user.size))
      |                             ^
make[2]: *** [/home/jlaitine/nuttx/boards/Board.mk:100: stm32_bbsram.o] Error 1
make[1]: *** [Makefile:196: board/libboard.a] Error 2
make: *** [tools/Unix.mk:569: nuttx] Error 2

With the corrections, the code compiles again.