powerpc: Add support for multiple gfar mdio interfaces

The old code assumed there was only one, but the 8572 actually has 3.

Also, our usual id, 0xe0024520, gets resolved to -1 somewhere, and this was
preventing the multiple buses from having different ids.  So we only keep
the low 20 bits, which have the interesting info, anyway.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>