[POWERPC] Add null pointer check to of_find_property

Update function of_find_property() to return NULL if the device_node
passed to it is also NULL.  Otherwise, passing NULL will cause a null
pointer dereference.

Without this, the legacy_serial driver will crash if there's no
'chosen' node in the device tree.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>