[POWERPC] bootwrapper: Add dt_is_compatible()

This can be used rather than doing a simple strcmp, which will fail to
handle multiple compatible entries.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>