ARM: renesas: Save boot parameters passed in by ATF

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. Save these registers for
future use.

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