dt-bindings: convert spmi.txt to spmi.yaml

Convert the SPMI bus documentation to JSON/yaml.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/ee4c4ca9f29a39f6af772b3a526a996176499da3.1598415179.git.mchehab+huawei@kernel.org


[robh: Correct maintainer, unit-address should be hex]
Signed-off-by: Rob Herring <robh@kernel.org>