ARM: meson: reset: Add reset controller for MesonX SoCs

This patch adds support for the reset controller found on the Amlogic
MesonX SoCs. For several devices in the AO (Always-On) power domain, it
is possible to reset them by programming a specific bit in a register.

Signed-off-by: Carlo Caione <carlo@caione.org>
ARM: meson: DTSI: Enable reset controller

Add the node for the reset controller in the DTSI.

Signed-off-by: Carlo Caione <carlo@caione.org>
ARM: meson: docs: Add docs for MesonX reset controller

Signed-off-by: Carlo Caione <carlo@caione.org>