usb: UniPhier: add UniPhier on-chip xHCI host driver support

Support xHCI host driver used on Panasonic UniPhier platform.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Marek Vasut <marex@denx.de>
diff --git a/doc/README.uniphier b/doc/README.uniphier
index aaeb50c..4902533 100644
--- a/doc/README.uniphier
+++ b/doc/README.uniphier
@@ -73,7 +73,8 @@
 
  - UART (on-chip)
  - NAND
- - USB (2.0)
+ - USB 2.0 (EHCI)
+ - USB 3.0 (xHCI)
  - LAN (on-board SMSC9118)
  - I2C
  - EEPROM (connected to the on-board I2C bus)