dt-bindings: net: marvell,orion-mdio: Fix examples

As stated in marvell-orion-mdio.txt deleted in commit 0781434a
("dt-bindings: net: orion-mdio: Convert to JSON schema") if
'interrupts' property is present, width of 'reg' should be 0x84.
Otherwise, width of 'reg' should be 0x4. Fix 'examples:' and add
constraints checking whether 'interrupts' property is present
and validate it against fixed values in reg.

Signed-off-by: Michał Grzelak <mig@semihalf.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>