mmc: uniphier: add driver for UniPhier SD/MMC host controller

Add a driver for the on-chip SD/eMMC host controller used by
UniPhier SoC family.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/doc/README.uniphier b/doc/README.uniphier
index f03c207..c270ecb 100644
--- a/doc/README.uniphier
+++ b/doc/README.uniphier
@@ -93,6 +93,7 @@
 
  - UART (on-chip)
  - NAND
+ - SD/eMMC
  - USB 2.0 (EHCI)
  - USB 3.0 (xHCI)
  - GPIO