dt-bindings: convert the binding document for mediatek PERICFG to yaml

Convert the DT binding .txt file for MediaTek's peripheral configuration
controller to YAML. There's one special case where the compatible has
three positions. Otherwise, it's a pretty normal syscon.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>