Convert CONFIG_OMAP_USB_PHY to Kconfig

This converts the following to Kconfig:
   CONFIG_OMAP_USB_PHY

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/cl-som-am57x_defconfig b/configs/cl-som-am57x_defconfig
index 9c3031b..e9b6fe0 100644
--- a/configs/cl-som-am57x_defconfig
+++ b/configs/cl-som-am57x_defconfig
@@ -57,6 +57,7 @@
 CONFIG_USB=y
 CONFIG_USB_XHCI_HCD=y
 CONFIG_USB_XHCI_DWC3=y
+CONFIG_OMAP_USB_PHY=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_HOST_ETHER=y
 CONFIG_USB_ETHER_ASIX=y