powerpc/boot: Fix compile warning in 64bit

arch/powerpc/boot/oflib.c:211:9: warning: cast to pointer from integer of \
		  different size [-Wint-to-pointer-cast]
  return (phandle) of_call_prom("finddevice", 1, 1, name);

This is a work around. The definite solution would be to define the
phandle typedef as a u32, as in the kernel, but this would break the
device tree ops API.

Let it be for the moment.

Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>