drivers: flash: stm32l5_u5: fix L5/U5 difference in FLASH_PAGE_NB

In flash_stm32_page_layout, the FLASH_PAGE_NB macro is used to set the
page count in the flash layout returned to the caller. On STM32L5
platforms this is defined in stm32l5xx_hal_flash.h to be (FLASH_SIZE /
FLASH_PAGE_SIZE). However, on STM32U5 platforms the macro is defined in
the soc headers (such as stm32u585xx.h) define the macro as
(FLASH_BANK_SIZE / FLASH_PAGE_SIZE). This results in the flash layout
only reflecting the first bank on STM32U5 platforms with the maximum 2
MB of flash.

This commit resolves this discrepancy by directly calculating the number
of pages by dividing FLASH_SIZE by FLASH_PAGE_SIZE.

Tested on a proprietary board using the STM32U585 which mounts a
littlefs partition in the last 256 kB of its 2 MB of flash.

Signed-off-by: Peter Maxwell Warasila <madmaxwell@soundcomesout.com>