ARM: dts: sun7i: Use most-qualified system control compatibles

This switches the sun7i-a20 dtsi to use the most qualified compatibles
for the system-control block (previously named SRAM controller) as well
as the SRAM blocks. The sun4i-a10 compatibles are kept since these
hardware blocks are backward-compatible.

The node name for system control is also updated to reflect the fact that
the controller described is really about system control rather than SRAM
control.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>