dt-bindings: reset: st,sti-powerdown: Convert to yaml

Convert the device tree bindings for the STMicroelectronics STi
Peripheral Powerdown reset controller to YAML schema to allow
participating in DT validation.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220407154338.4190674-13-p.zabel@pengutronix.de