ARM: renesas: Rename rmobile_cpuinfo_idx() to renesas_cpuinfo_idx()

Rename rmobile_cpuinfo_idx() to renesas_cpuinfo_idx() because all
the chips are made by Renesas, while only a subset of them is from
the R-Mobile line.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>