mtd: devices: add devicetree documentation for microchip 48l640

The Microchip 48l640 is a 8KByte EERAM connected via SPI.
Add devicetree bindings documentation.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210607033909.1424605-2-hs@denx.de