dt-bindings: net: cdns,macb: Convert to json-schema

Convert the Cadence MACB/GEM Ethernet controller Device Tree binding
documentation to json-schema.

Re-add "cdns,gem" (removed in commit a217d871 ("dt-bindings:
Remove PicoXcell bindings")) as there are active users on non-PicoXcell
platforms.
Add missing "ether_clk" clock.
Add missing properties.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/104dcbfd22f95fc77de9fe15e8abd83869603ea5.1637927673.git.geert@linux-m68k.org


Signed-off-by: Rob Herring <robh@kernel.org>