davinci: mityomapl138: Use auto-probe to determine attached PHY ID

Current board configurations involving the MityDSP-L138 and MityARM-1808
only have one attached PHY, but it's address may not be the same.  Default
the behavior to auto-probe for the PHY and use the first one found.

Signed-off-by: Michael Williamson <michael.williamson@criticallink.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>