pinctrl: baytrail: add missing module removal support

pinctrl-baytrail driver provides a proper ->remove()
method on its platform_driver definition, however there's
no way, currently, to unload the driver due to missing
module_exit(). This patch adds module_exit().

Signed-off-by: Felipe Balbi <balbi@ti.com>
Acked-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>