pinctrl: ab8500: Staticize some symbols

These symbols are used only in this file. Silences the following
warnings:
drivers/pinctrl/pinctrl-ab8500.c:392:28: warning:
symbol 'ab8500_alternate_functions' was not declared. Should it be static?
drivers/pinctrl/pinctrl-ab8500.c:458:32: warning:
symbol 'ab8500_gpio_irq_cluster' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>