soc: renesas: rcar-rst: Add R8A77961 support

Add support for the Reset block in the R-Car M3-W+ (R8A77961) SoC to the
Renesas R-Car RST driver.

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/20191023123342.13100-6-geert+renesas@glider.be