mfd: Fix build breakage in omap-usb-host.c

'ARM: OMAP3: USB: Fix the EHCI ULPI PHY reset issue' removes the include for
linux/gpio.h from omap-usb-host.c. This include indirectly includes plat/cpu.h
which is required by omap-usb-host.c. Fix the build breakage by including
it directly.

Acked-by: Keshava Munegowda <keshava_mgowda@ti.com>
Acked-by: Kevin Hilman <khilman@ti.com>
Tested-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Russ Dill <Russ.Dill@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>