drivers: flash : stm32h7: Fix the offset for get_sector()

The offset is wrong when the second bank is used by M4 as the
offset is relative to the base which is 0. So add the least
significant 6 nibbles from the CONFIG_FLASH_BASE_ADDRESS to
find the sector.

Signed-off-by: Murali Karicheri <murali.karicheri@sandc.com>