soc: renesas: Identify R-Car M3-W+

Add support for identifying the R-Car M3-W+ (R8A77961) SoC, which shares
the Product ID Number with R-Car M3-W (R8A77960), but differs in CUT
Number (Ver. 3.0), and uses a different compatible value.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20191023123342.13100-5-geert+renesas@glider.be