Reserve 4 kiB of space in 64bit R-Car DTs when those DTs are compiled
to permit patching in OpTee-OS /firmware node, /reserved-memory node,
possibly also additional /memory@ nodes and RPC node by TFA.

This duplicates behavior in arch/arm/dts/Makefile with OF_UPSTREAM.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>