dt-bindings: net: add reset property for aspeed, ast2600-mdio binding

The AST2600 MDIO bus controller has a reset control bit and must be
deasserted before manipulating the MDIO controller. By default, the
hardware asserts the reset so the driver only need to deassert it.

Regarding to the old DT blobs which don't have reset property in them,
the reset deassertion is usually done by the bootloader so the reset
property is optional to work with them.

Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>