pinctrl: sprd: make three local functions static

The functions sprd_pmx_get_function_count, sprd_pmx_get_function_name
and sprd_pmx_get_function_groups are local to the source and do not
need to be in global scope, so make them static.

Cleans up sparse warnings:
"symbol 'sprd_pmx_get_function_count' was not declared. Should it be
static?"
"symbol 'sprd_pmx_get_function_name' was not declared. Should it be
static?"
"symbol 'sprd_pmx_get_function_groups' was not declared. Should it be
static?"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>