ARM: rmobile: Fix the memory map on Gen3

Fix up the memory map on Gen3 to match datasheet properly.
This simplifies the memory map setup as well, since we do
no longer need this massive complexity.

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