ARM: dts: sun8i: Add R_CPUCFG device node for the A83T dtsi

The R_CPUCFG is a collection of registers needed for SMP bringup
on clusters and cluster's reset.
For the moment, documentation about this register is found in
Allwinner's code only.

Signed-off-by: Mylène Josserand <mylene.josserand@bootlin.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>