drivers: net: cpsw: add support to update phy address

drivers: net: cpsw: add support to update phy address

On some boards using TI CPSW, it may be possible that
PHY address was not latched correctly, and the actual
address that the phy responds on is different from that
set in device-tree. For example, see this problem report
on beaglebone black:

https://groups.google.com/d/msg/beagleboard/9mctrG26Mc8/1FuI_i5KW10J

Add support to check for this condition and use the
detected phy address when its safe to do so.

Also, add a public API that exposes the phy address of
a given slave. This can be used to update device-tree that
is passed to Linux kernel.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>