ARM: dts: imx6q-tbs2910: Pass reset-assert-us

According to the AR8035 datasheet:

"When using crystal, the clock is generated internally after power is
stable. For a reliable power on reset, suggest to keep asserting the reset
low long enough (10ms) to ensure the clock is stable and clock-to-reset 1ms
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>