drivers: flash: Add support for Atmel AT25 SPI flash variant

The AT25XV021A variant is a flash variant of Atmel's AT25 family
that adds extra protections, requiring additional writes to the
device to program or erase data.

This commit adds a flash driver for AT25XV021A devices instead of
modifying (1) the existing AT45 SPI flash driver or (2) the
existing AT24/25 EEPROM driver because this variant poses
fundamental changes that affect all aspects of the driver.

Notably,
 - AT25XV021A includes a second status register, and the format
	and functions of the existing status register is
	changed from the existing drivers.
 - AT25XV021A requires executing page or chip erase commands
	before writing, making it incompatible with the
	existing AT24/25 EEPROM driver.
 - AT25XV021A adds a software protection layer that requires
	extra writes before executing program or erase commands.

Tested writing to and erasing from an AT25XV021A device. Tested
reading from an AT25XV021A device across page boundaries. Tested
chip erase function. Tested driver initialization from varying
initial hardware states.

Signed-off-by: Liam Ogletree <liam.ogletree@cirrus.com>