soc: renesas: Add ARCH_R8A77960 for existing R-Car M3-W

Add CONFIG_ARCH_R8A77960 as a new config symbol for R-Car M3-W
(R8A77960), to replace CONFIG_ARCH_R8A7796, and avoid confusion with
R-Car M3-W+ (R8A77961), which will use CONFIG_ARCH_R8A77961.

Note that for now, CONFIG_ARCH_R8A7796 is retained, and just selects
CONFIG_ARCH_R8A77960.  This relaxes dependencies of other subsystems on
the SoC configuration symbol, and provides a smooth transition path for
config files through "make oldconfig".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20191023123342.13100-3-geert+renesas@glider.be