[POWERPC] Document phy-connection-type property

Since ucc_geth is being migrated to use the phylib, the existing
(undocumented) 'interface' property is being deprecated in favour
of 'phy-connection-type'.

phy-connection-type is now maintained one-to-one with definitions
in include/linux/phy.h, albeit in the form of a string.
If not specified, "mii" is assumed.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>