drivers: flash: flexspi: Add 4-byte addressing support for MT25 family

Currently, MT25 flashes were running in 3-byte mode.
This is not compatible with the chip we use in our project (MT25QU01GBBB),
as only 128 Mbit of its 1 Gbit can be addressed.

Signed-off-by: Martin Stumpf <finomnis@gmail.com>