mtd: spi-nor: Add support for mr25h40

Add Everspin mr25h40 512KB MRAM to the list of supported chips.

Signed-off-by: Masahiko Iwamoto <iwamoto@allied-telesis.co.jp>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>