dt-bindings: clock: add mtmips SoCs system controller

Adds device tree binding documentation for system controller node present
in Mediatek MIPS and Ralink SOCs. This node is a clock and reset provider
for the rest of the world. This covers RT2880, RT3050, RT3052, RT3350,
RT3883, RT5350, MT7620, MT7628 and MT7688 SoCs.

Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>