dt-bindings: bus: add palmbus device tree bindings

Add device tree bindings for palmbus controller present in all the MIPS
ralink based SoCs.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20211016203323.9165-1-sergio.paracuellos@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>