drivers: flash: flash_stm32l5x: fix the STM32U5X flash bank issue

STM32U5X has 128k/256k/512k/1M/2M dual bank Flash.
The address of the 2 bank are continuous, so it's no need a "Dummy page"
in "stm32_flash_layout", which cause wrong slot1 section (for secondary
image), and the BANK2_OFFSET is not right either, which cause
"flash_stm32_valid_range" return a failure.
To fix the issue, just set CONFIG_FLASH_SIZE to STM32_SERIES_MAX_FLASH
Tested on NUCLEO-U545RE with mcuboot.

Signed-off-by: Weifeng Li <weifeng.li@aofrio.com>