UnevaluatedProperties and DWMAC

Hi Rob,

Thanks for updating jsonschema and supporting the 2019.09 Draft. I tried
to do this recently but struggled a bit.

As you can imagine, I went along and ran it on the Allwinner SoCs and it
looks like there's a weird interaction between unevaluatedProperties and
Documentation/devicetree/bindings/net/snps,dwmac.yaml

Indeed, schemas using it (allwinner,sun7i-a20-gmac.yaml is a rather
straightforward example) seem to fail the unevaluatedProperties check,
like:

.../arch/arm/boot/dts/sun7i-a20-wexler-tab7200.dt.yaml: ethernet@1c50000: Unevaluated properties are not allowed ('snps,pbl', 'snps,fixed-burst', 'snps,force_sf_dma_mode', 'mdio' were unexpected)
	From schema: .../Documentation/devicetree/bindings/net/allwinner,sun7i-a20-gmac.yaml

All these properties are definitely in snps,dwmac.yaml, and it's pretty
much the only issue when running dtbs_check across all the sunxi DTs, so
unevaluatedProperties seems to be working just fine as well.

I first wondered if it could be because of the custom select clause in
snps,dwmac.yaml that doesn't include the allwinner,sun7i-a20-gmac
compatible, but even removing that clause entirely doesn't improve the
situation.

Do you have an idea of what could be going on?

Thanks!
Maxime