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 <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>