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/configs/firefly-rk3288_defconfig b/configs/firefly-rk3288_defconfig
index e9eb20c..3a0bd79 100644
--- a/configs/firefly-rk3288_defconfig
+++ b/configs/firefly-rk3288_defconfig
@@ -71,6 +71,7 @@
 CONFIG_SYSRESET=y
 CONFIG_USB=y
 CONFIG_USB_DWC2=y
+CONFIG_ROCKCHIP_USB2_PHY=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_KEYBOARD=y
 CONFIG_USB_GADGET=y