ARM: rmobile: Unify memory map for RCar Gen3

ARM: rmobile: Unify memory map for RCar Gen3

Unify the R7A7795 and R8A7796 memory maps in memmap-gen3 and, for now,
select which one is used based on which SoC is selected. Since this is
done in C code instead of statically assigned now, the decision can be
taken by PRR SoC match as well, which will be done in a subsequent patch.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>