gpio: pch: Build context save/restore only for PM

The pch_gpio_save_reg_conf() and pch_gpio_restore_reg_conf() functions
are only used in pch_gpio_suspend() and pch_gpio_resume(), respectively.
Since the latter are only built if PM is enabled, make the former build
under the same conditions.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>