mtd: spi-nor: Move Micron/ST bits out of core.c

Create a SPI NOR manufacturer driver for Micron/ST chips, and move the
Micron/ST definitions outside of core.c.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>