mtd: m25p80: add support for Everspin MR25H256 MRAM chip

This chip isn't JEDEC-compatible.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>