gpiolib-of: staticize the pin range calls

Commit 69e1601bca88809dc118abd1becb02c15a02ec71
"gpiolib: provide provision to register pin ranges"

Declared the of_gpiochip_[add|remove]_pin_range() global
while they should be static as they are only ever used in
this file. Let's convert them to static.

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