ARM: shmobile: pm-rmobile: Use GENPD_FLAG_ALWAYS_ON

Improve handling of always-on PM domains by using the
GENPD_FLAG_ALWAYS_ON flag introduced in commit ffaa42e8

 ("PM /
Domains: Enable users of genpd to specify always on PM domains").

Note that the PM domain containing the serial console is still handled
locally.

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