mtd: devices: add support for microchip 48l640 EERAM

The Microchip 48l640 is a 8KByte EERAM connected via SPI.

Signed-off-by: Heiko Schocher <hs@denx.de>
Tested-by: Fabio Estevam <festevam@denx.de>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210607033909.1424605-3-hs@denx.de