gpio: omap: Save and restore sysconfig

As we are using cpu_pm to save and restore context, we must also save and
restore the GPIO sysconfig register. This is needed because we are not
calling PM runtime functions at all with cpu_pm.

We need to save the sysconfig on idle as it's value can get reconfigured by
PM runtime and can be different from the init time value. Device specific
flags like "ti,no-idle-on-init" can affect the init value.

Fixes: b764a586 ("gpio: omap: Remove custom PM calls and use cpu_pm instead")
Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Adam Ford <aford173@gmail.com>
Cc: Andreas Kemnade <andreas@kemnade.info>
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: Peter Ujfalusi <peter.ujfalusi@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>