mmc: renesas_sdhi: Add MODULE_LICENSE

The following error occurs when loading renesas_sdhi_core.c module,
so add MODULE_LICENSE("GPL v2").

 renesas_sdhi_core: module license 'unspecified' taints kernel.

Signed-off-by: Masaharu Hayakawa <masaharu.hayakawa.ry@renesas.com>
Fixes: 9d08428a

 ("mmc: renesas-sdhi: make renesas_sdhi_sys_dmac main module file")
Cc: <stable@vger.kernel.org> # v4.13+
[Shimoda: Added Fixes tag and Cc to the stable ML]
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>