gpio: stmpe: Staticize non-exported symbols

Both stmpe_gpio_irq_map() and stmpe_gpio_irq_unmap() are not referenced
outside of this file, make them static.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>