ARM: dts: imx6qdl-sabresd: Pass reset-assert-us

According to the AR8031 datasheet:

"When using crystal, clock is generated internally after the power is
stable. In order to get reliable power-on-reset, it is recommended to
keep asserting the reset low signal long enough (10 ms) to ensure the
clock is stable and clock-to-reset (1 ms) requirement is satisfied."

Pass the 'reset-assert-us' property to describe such requirement.

While at it, use the 'reset-gpios' property inside the the mdio
node instead of the deprecated usage of 'phy-reset-gpios'.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Soeren Moch <smoch@web.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>