[POWERPC] Document the mv64x60 device tree bindings

Add the device tree bindings for the Marvell mv64x60 series of
system controller chips in booting-without-of.text.

Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Acked-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>