mmc: sdhci-of-arasan: Add the support for sdhci-5.1

This patch adds the compatible string in sdhci-of-arasan.c to
support sdhci-arasan5.1 version of controller. No documented
controller IP version is found in the TRM, so we use ths version
of command queueing engine integrated into this controller by arasan
to specify our controller.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>