pinctrl: spear/plgpio: Staticize spear310_o2p

'spear310_o2p' is referenced only in this file.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>