soc: renesas: rmobile-sysc: Remove unneeded platform includes

As of commit b5872880 ("ARM: shmobile: R-Mobile: Remove legacy
PM Domain code"), the R-Mobile System Controller driver no longer
handles the adding of platform devices to PM Domains, but delegates that
to the PM Domain core code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20210205132141.1920137-1-geert+renesas@glider.be