mtd: spi-nor: Add Global Block Unlock command

The Global Block Unlock command has different names depending
on the manufacturer, but always the same command value: 0x98.
Macronix's MX25U12835F names it Gang Block Unlock, Winbond's
W25Q128FV names it Global Block Unlock and Microchip's
SST26VF064B names it Global Block Protection Unlock.

Used in the Individual Block Protection mode, which is mutually
exclusive with the Block Protection mode (BP0-3).

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20210121110546.382633-1-tudor.ambarus@microchip.com