pinctrl: core: Add missing EXPORT on pinctrl_register_mappings

Systems that don't have devicetree need pinctrl_register_mappings.
It should be EXPORT_SYMBOL_GPL so that it can be called from
pinctrl drivers built as modules.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>