device_tree: load_device_tree(): Allow NULL sizep

The sizep arg is populated with the size of the loaded device tree. Since this
is one of those informational "please populate" type arguments it should be
optional. Guarded writes to *sizep against NULL accordingly.

Signed-off-by: Peter A. G. Crosthwaite <peter.crosthwaite@petalogix.com>
Acked-by: Alexander Graf <agraf@suse.de>