drivers: flash: Add generic NOR flash driver for MSPI devices

Add a flash driver intended to handle various flash devices
connected over MSPI bus as long as they support JEDEC SFDP.
This is an initial commit providing only basic operations
in Octal I/O mode with some hard-coded values for Macronix
MX25Ux series chips.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>