soc: renesas: rcar-rst: add R8A77970 support

Add support for R-Car V3M (R8A77970) to the R-Car RST driver -- this driver
is  needed  for the clock driver to work.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>