mfd: omap-usb: Depend on COMMON_CLK to fix compile tests

The MFD_OMAP_USB_HOST uses Common Clock Framework thus it cannot be
built on platforms without it (e.g. compile test on MIPS with LANTIQ):

mips-linux-ld: drivers/mfd/omap-usb-host.o: in function `usbhs_omap_probe':
omap-usb-host.c:(.text+0x940): undefined reference to `clk_set_parent'

Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/Kconfig

···
 config MFD_OMAP_USB_HOST
 	bool "TI OMAP USBHS core and TLL driver"
 	depends on USB_EHCI_HCD_OMAP || USB_OHCI_HCD_OMAP3
+	depends on COMMON_CLK
 	default y
 	help
 	  This is the core driver for the OAMP EHCI and OHCI drivers.