gpio: uniphier: add driver for UniPhier GPIO controller

This GPIO controller device is used on UniPhier SoCs.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/README.uniphier b/doc/README.uniphier
index bcf0ac3..f03c207 100644
--- a/doc/README.uniphier
+++ b/doc/README.uniphier
@@ -95,6 +95,7 @@
  - NAND
  - USB 2.0 (EHCI)
  - USB 3.0 (xHCI)
+ - GPIO
  - LAN (on-board SMSC9118)
  - I2C
  - EEPROM (connected to the on-board I2C bus)