ARM: dts: rockchip: add RK3288 suspend support

add pmu sram node for suspend, add global_pwroff pinctrl.
The pmu sram is used to store the resume code.
global_pwroff is held low level at work, it would be pull to high
when entering suspend. reference this in the board DTS file since
some boards need it.

Signed-off-by: Tony Xie <xxx@rock-chips.com>
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Tested-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>