drivers: flash : stm32: STM32L4P5xx fix page calculation from offset

This fix adds the STM32L4P5xx to the list of devices that have an
offset-to-page shift calculation of 12 bits. Previously, the driver
would only shift the offset by 11 bits when calculating the page to
erase.

This would prevent the driver from erasing the correct page.

Signed-off-by: George Beckstein <george.beckstein@ampaworks.com>