mtd: spi-nor: Split mt25qu512a (n25q512a) entry into two

mt25q family is different from n25q family of devices, even though manf
ID and device IDs are same. mt25q flash has bit 6 set in 5th byte of
READ ID response which can be used to distinguish it from n25q variant.
mt25q flashes support stateless 4 Byte addressing opcodes where as n25q
flashes don't. Therefore, have two separate entries for mt25qu512a and
n25q512a.

Fixes: 9607af6f ("mtd: spi-nor: Rename "n25q512a" to "mt25qu512a (n25q512a)"")
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>