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/rock_defconfig b/configs/rock_defconfig
index b174e4b..483f64b 100644
--- a/configs/rock_defconfig
+++ b/configs/rock_defconfig
@@ -47,6 +47,8 @@
 CONFIG_DEBUG_UART_SHIFT=2
 CONFIG_SYS_NS16550=y
 CONFIG_SYSRESET=y
+CONFIG_USB=y
+CONFIG_ROCKCHIP_USB2_PHY=y
 CONFIG_SPL_TINY_MEMSET=y
 CONFIG_TPL_TINY_MEMSET=y
 CONFIG_CMD_DHRYSTONE=y