pinctrl: pinctrl_register_mappings() shouldn't be __init

It may be common for pinctrl_register_mappings() to be used from __init
context, but there's no reason that additional mappings shouldn't be
added at a later point, e.g. if loading modules that add pin controllers
and their mapping tables.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>