mtd: spi-nor: core: Introduce spi_nor_set_4byte_addr_mode()

Make the method public, as it will be used as a last resort to enable
4byte address mode when we can't determine the address mode at runtime.
Update the addr_nbytes and current address mode while exiting the 4byte
address mode too, as it may be used in the future by manufacturer
drivers. No functional change. spi_nor_restore didn't update the address
mode nbytes, but updating them now doesn't harm as the method is called
in the driver's remove and shutdown paths.

Link: https://lore.kernel.org/r/20230331074606.3559258-10-tudor.ambarus@linaro.org
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>