mmc: sdhci-cadence: add Socionext UniPhier specific compatible string

Add a Socionext SoC specific compatible (suggested by Rob Herring).

No SoC specific data are associated with the compatible strings for
now, but other SoC vendors may use this IP and want to differentiate
IP variants in the future.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>