phy: fix Kconfig dependencies

DM816x PHY uses usb_phy_* methods and because
of that, it must select USB_PHY, however, because
the drivers in question (DM816x, TWL4030 and
OMAP_USB2) sit outside of drivers/usb/ directory,
meaning they can be built even if USB_SUPPORT=n.

This patches fixes the dependencies by adding
USB_SUPPORT as a dependency and making all drivers
select USB_PHY (which cannot be selected through
menuconfig).

Note that this fixes some linking breakages when
building with randconfig.

Cc: Tony Lindgren <[e-mail]>
Cc: Kishon Vijay Abraham I <[e-mail]>
Acked-by: Tony Lindgren <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>
Signed-off-by: Kishon Vijay Abraham I <[e-mail]>

drivers/phy/Kconfig

···
 config PHY_DM816X_USB
 	tristate "TI dm816x USB PHY driver"
 	depends on ARCH_OMAP2PLUS
+	depends on USB_SUPPORT
 	select GENERIC_PHY
+	select USB_PHY
 	help
 	  Enable this for dm816x USB to work.
 
···
 config OMAP_USB2
 	tristate "OMAP USB2 PHY Driver"
 	depends on ARCH_OMAP2PLUS
-	depends on USB_PHY
+	depends on USB_SUPPORT
 	select GENERIC_PHY
+	select USB_PHY
 	select OMAP_CONTROL_PHY
 	depends on OMAP_OCP2SCP
 	help
···
 config TWL4030_USB
 	tristate "TWL4030 USB Transceiver Driver"
 	depends on TWL4030_CORE && REGULATOR_TWL4030 && USB_MUSB_OMAP2PLUS
-	depends on USB_PHY
+	depends on USB_SUPPORT
 	select GENERIC_PHY
+	select USB_PHY
 	help
 	  Enable this to support the USB OTG transceiver on TWL4030
 	  family chips (including the TWL5030 and TPS659x0 devices).