mtd: spi-nor: add dt support for Everspin MRAMs

The MR25 family doesn't support JEDEC, so they need explicit mentioning
in the list of supported spi IDs. This makes it possible to add these
using for example:

	compatible = "everspin,mr25h40";

There was already an entry for mr25h256. Move that one out of the "keep
for compatibility" section and put in a new group for Everspin MRAMs.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>