clk: renesas: mstp: Keep wakeup sources active during system suspend

If a device is part of the CPG/MSTP Clock Domain and to be used as a
wakeup source, it must be kept active during system suspend.

Currently this is handled in device-specific drivers by explicitly
increasing the use count of the module clock when the device is
configured as a wakeup source.  However, the proper way to prevent the
device from being stopped is to inform this requirement to the genpd
core, by setting the GENPD_FLAG_ACTIVE_WAKEUP flag.

Note that this will only affect devices configured as wakeup sources.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>