clk: renesas: Rename renesas-rzg2l-cpg.[ch] to rzg2l-cpg.[ch]

Rename renesas-rzg2l-cpg.c and renesas-rzg2l-cpg.h to rzg2l-cpg.c resp.
rzg2l-cpg.h, for consistency with other (sub)drivers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/edc442daaedffcf10e835ff479d906fcae0e59db.1626268821.git.geert+renesas@glider.be