Convert CONFIG_ROCKCHIP_USB2_PHY to Kconfig

This converts the following to Kconfig:
   CONFIG_ROCKCHIP_USB2_PHY

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/drivers/usb/phy/Kconfig b/drivers/usb/phy/Kconfig
index a202fd1..bcc67a0 100644
--- a/drivers/usb/phy/Kconfig
+++ b/drivers/usb/phy/Kconfig
@@ -13,3 +13,5 @@
 config OMAP_USB_PHY
 	bool "OMAP PHY"
 
+config ROCKCHIP_USB2_PHY
+	bool "Rockchip USB2 PHY"