[POWERPC] Add dt_xlate_addr() to bootwrapper

dt_xlate_reg() looks up the 'reg' property in the specified node
to get the address and size to translate.  Add dt_xlate_addr()
which is passed in the address and size to translate.

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