[POWERPC] prpmc2800: Clean up dts properties

Remove several unused (or software config only) properties.
Rename marvel node to "system-controller".  Also, rename the
"block-index" property to "cell-index" to conform to current
practice.

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