[POWERPC] wrapper: Rename offset in offset_devp()

fdt_wrapper_create_node passes a variable called offset to offset_devp(),
which uses said parameter to initialize a local variable called offset.

Due to one of the odder aspects of the C language, the result is an
undefined variable, with no error or warning.

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>