ARM: rockchip: Add pmu-sram binding

The pmu-sram is used to store resume code, suspend/resume need get the
address of it. Therefore add a binding and documentation for 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>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>