ARM: renesas: Configure DRAM size from ATF DT fragment

The ATF can pass additional information via the first four registers,
x0...x3. The R-Car Gen3 with mainline ATF, register x1 contains pointer
to a device tree with platform information. Parse this device tree and
extract DRAM size information from it. This is useful on systems where
the DRAM size can vary between configurations.

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