soc: renesas: rcar-sysc: Make rcar_sysc_init() init the PM domains

Let rcar_sysc_init() trigger initialization of the SYSC PM domains from
DT if called before the early_initcall.
On failure, it falls back to mapping the passed register block, as
before.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>