dt-bindings: net: smsc,lan91c111 convert to schema

Convert the smsc lan91c9x and lan91c1xx controller device tree
bindings documentation to json-schema.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220331154536.1544220-2-rui.silva@linaro.org