dt-bindings: mmc: Convert pwrseq to json-schema

Convert the pwrseq binding to DT schema format using json-schema.

At the same time, fix a couple of issues with the examples discovered by
the validation tool -- missing ";"

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200622164431.3dbc8c5a@xhacker.debian


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>