mtd: rawnand: sh_flctl: convert to SPDX identifiers

This patch updates license to use SPDX-License-Identifier
instead of verbose license text.

As original license mentioned, it is GPL-2.0 in SPDX.
Then, MODULE_LICENSE() should be "GPL v2" instead of "GPL".
See ${LINUX}/include/linux/module.h

	"GPL"		[GNU Public License v2 or later]
	"GPL v2"	[GNU Public License v2]

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>