proc_devtree: include linux/of.h

Currenly, proc_devtree.c depends on asm/prom.h to include linux/of.h, to
provide some device-tree definitions (eg, struct property).

Instead, include linux/of.h directly. We still need asm/prom.h for
HAVE_ARCH_DEVTREE_FIXUPS.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>