ARM: dts: rmobile: Reserve space in R-Car Gen3 DTs

Reserve 4 kiB of space in R-Car Gen3 DTs when those DTs are compiled
to permit patching in OpTee-OS /firmware node, /reserved-memory node
and possibly also additional /memory@ nodes.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>