flash: stm32h7x: fix implicit declaration for LL_GetFlashSize

Since https://github.com/zephyrproject-rtos/zephyr/pull/83114 was merged,
I get twister errors unrelated to my pull requests for the STM32H7S78-DK
(for example https://github.com/zephyrproject-rtos/zephyr/actions/runs/14900504138/job/41851537049#step:12:1369)

This is because the function LL_GetFlashSize() was removed from hal_stm32
on the STM32H7RS series in
https://github.com/zephyrproject-rtos/hal_stm32/commit/e5eba65b7674d2883817203856b1d5eb8d08132d



I am unsure if the removal from hal_stm32 is intentional. If yes, then this
commit should be merged into Zephyr to fix compilation for that target.
Otherwise, hal_stm32 must be patched, Zephyr's west manifest updated, and
this commit can be discarded.

Signed-off-by: Titouan Christophe <titouan.christophe@mind.be>