soc: renesas: rcar-sysc: Mark rcar_sysc_matches[] __initconst

rcar_sysc_matches[] is used only by rcar_sysc_pd_init(), which is
__init.  Hence mark rcar_sysc_matches[] __initconst.

This frees another 1764 bytes (arm32/shmobile_defconfig) or 1000 bytes
(arm64/renesas_defconfig) of memory after kernel init.

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