ARM: shmobile: update SDHI DT compatibility string to the <unit>-<soc> format

Currently DT compatibility strings of both types can be found in the kernel
sources: <unit>-<soc> and <soc>-<unit>, whereas a unique format should be
followed and the former one is preferred. This patch converts the SDHI
MMC driver and its users to the common standard. This is safe for now, since
ATM no real products are using this driver with DT.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Acked-by: Chris Ball <cjb@laptop.org>
[Removed r8a7740.dtsi portion as it is not applicable]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>