dt-bindings: Add compatibles for undocumented trivial syscons

'intel,lgm-syscon', 'marvell,armada-3700-usb2-host-misc',
'mediatek,mt8135-pctl-a-syscfg', and 'mediatek,mt8135-pctl-b-syscfg' are
all used in DT binding examples and/or dts files, but are not
documented. Add them to syscon.yaml as they are all trivial cases.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220228221537.1700071-1-robh@kernel.org