[PATCH] powerpc: Add FSL USB node to documentation

Updated the documentation to include the definition of the USB device
node format for Freescale SOC devices.

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