mx6sabresd: Simplify the Ethernet PHY configuration

As per the AR8031 datasheet:

"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."

So do as suggested and also add a 100us delay after deasserting the
reset line to guarantee that the PHY ID can be read correctly and the
Atheros 8031 PHY driver can be loaded automatically.

This results in a simpler code.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>