ramips: add support for switching between 3-byte and 4-byte addressing on w25q256 flash

On some devices the flash chip needs to be in 3-byte addressing mode during
reboot, otherwise the boot loader will fail to start.
This mode however does not allow regular reads/writes onto the upper 16M
half. W25Q256 has separate read commands for reading from >16M, however
it does not have any separate write commands.
This patch changes the code to leave the chip in 3-byte mode most of the
time and only switch during erase/write cycles that go to >16M
addresses.

Signed-off-by: Felix Fietkau <nbd@nbd.name>