soc: renesas: rmobile-sysc: Set GENPD_FLAG_ALWAYS_ON for always-on domain

Currently the R-Mobile "always-on" PM Domain is implemented by returning
-EBUSY from the generic_pm_domain.power_off() callback, and doing
nothing in the generic_pm_domain.power_on() callback.  However, this
means the PM Domain core code is not aware of the semantics of this
special domain, leading to boot warnings like the following on
SH/R-Mobile SoCs:

    sh_cmt e6130000.timer: PM domain c5 will not be powered off

Fix this by making the always-on nature of the domain explicit instead,
by setting the GENPD_FLAG_ALWAYS_ON flag.  This removes the need for the
domain to provide power control callbacks.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>