loader: Add load_image_size() to replace load_image()

A subsequent patch to ppc/spapr needs to load the RTAS blob into
qemu memory rather than target memory (so it can later be copied
into the right spot at machine reset time).

I would use load_image() but it is marked deprecated because it
doesn't take a buffer size as argument, so let's add load_image_size()
that does.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[aik: fixed errors from checkpatch.pl]
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
