plat-nomadik: get rid of unused GPIO PM code

The NOMADIK_GPIO_PM config option is disabled by default, not user visible,
and never selected by any other option: the code is therefore unused. The
GPIO registers need not be saved and restored since their values are
preserved when vAPE (on DB8500) is powered down.

Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Reviewed-by: Jonas Aberg <jonas.aberg@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>