powerpc: Add missing declaration in asm/drmem.h

Don't rely on random inclusion of linux/of.h by users
of asm/drmem.h

Add a forward declaration of struct property and
struct device_node.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5643ec410e51b749db0636471cb7979524f9ed0e.1646767214.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/drmem.h

@@ -23,6 +23,9 @@ struct drmem_lmb_info {
 	u64                     lmb_size;
 };
 
+struct device_node;
+struct property;
+
 extern struct drmem_lmb_info *drmem_info;
 
 static inline struct drmem_lmb *drmem_lmb_next(struct drmem_lmb *lmb,