dt-bindings: ARM: Mediatek: Document devicetree bindings for clock/reset controllers

This adds the binding documentation for the apmixedsys, perisys and
infracfg controllers found on Mediatek SoCs.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>