[PATCH] documentation/powerpc: add bus-frequency property to SOC node

Updated SOC node definition in documentation to include bus-frequency
property. Also extended mdio example to match specification.

Signed-off-by: Becky Bruce <becky.bruce@freescale.com>
Signed-off-by: Kumar Gala <galak@gate.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>