[ARM] pxa/spitz: provide spitz_ohci_exit() that unregisters USB_HOST GPIO

Currently spitz_ohci_init() that requests GPIO doesn't have
corresponding spitz_ohci_exit() which will gpio_free(). This causes
minor problems e.g. during resume when the OHCI device can't be resumed.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Eric Miao <eric.miao@marvell.com>