msm: gpiomux: Move public API to public header

The gpiomux.h header contains some SOC ifdefs.  However, the API that
is actually used by the GPIO driver only uses two functions that are
general.  Move these general definitions into a public header file.

Change-Id: Ia5df8af87dba268225598d56908e523bcfc24ef6
Signed-off-by: David Brown <davidb@codeaurora.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>