mtd: spi-nor: Add support for XM25QH64A and XM25QH128A

These devices are produced by Wuhan Xinxin Semiconductor Manufacturing
Corp. (XMC) and found on some routers from Chinese manufactures.

The data sheets can be found here:
http://www.xmcwh.com/Uploads/2018-03-01/5a9799e4cb355.pdf
http://www.xmcwh.com/Uploads/2018-02-05/5a77e6dbe968b.pdf



Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>