drivers: net: cpsw: Add reading of DT phy-handle node

Add the ability to read the phy-handle node of the
cpsw slave.  Upon reading this handle the phy-id
can be stored based on the reg node in the DT.

The phy-handle also needs to be stored and passed
to the phy to access any phy data that is available.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Tested-by: Mugunthan V N <mugunthanvnm@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>