pinctrl: pinctrl-single: Add functions to save and restore pinctrl context

This adds a pair of context save/restore functions to save/restore the
state of a set of pinctrl registers. The context is lost during rtc only
suspend with ddr in self-refresh on am43xx. Currently the save/restore
is being done unconditionally. This will be optimized later with a
pdata-quirk function which will allow is to save/restore only when doing
the rtc only mode with ddr in self refresh.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>