arm64: dts: uniphier: add SD-ctrl node for LD11 SoC

The LD11 SoC is equipped with SD-ctrl (0x59810000) as well as
MIO-ctrl (0x5b3e0000).  The SD-ctrl block on this SoC has just
one register for controlling RST_n pin of the eMMC device.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>