soc: renesas: Add ARCH_R8A7795[01] for existing R-Car H3

Despite using the same compatible values ("r8a7795"-based) because of
historical reasons, R-Car H3 ES1.x (R8A77950) and R-Car H3 ES2.0+
(R8A77951) are really different SoCs, with different part numbers.

Reflect this in the SoC configuration, by adding CONFIG_ARCH_R8A77950
and CONFIG_ARCH_R8A77951 as new config symbols.  These are intended to
replace CONFIG_ARCH_R8A7795, and will allow making support for early SoC
revisions optional.

Note that for now, CONFIG_ARCH_R8A7795 is retained, and just selects
CONFIG_ARCH_R8A77950 and CONFIG_ARCH_R8A77951.  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>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20191217183841.432-6-geert+renesas@glider.be