Extract out a portion of nmk_gpio_set_mode() that can be called with the
lock already held.  This will be used from the pincfg API.

Cc: Alessandro Rubini <rubini@unipv.it>
Acked-by: NLinus Walleij <linus.walleij@stericsson.com>
Signed-off-by: NRabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>