metag: prom.h: remove declaration of metag_dt_memblock_reserve()

Metag doesn't have a metag_dt_memblock_reserve() function so remove the
declaration from asm/prom.h.
Signed-off-by: James Hogan <james.hogan@imgtec.com>

arch/metag/include/asm/prom.h

@@ -19,6 +19,5 @@
 
 extern struct machine_desc *setup_machine_fdt(void *dt);
 extern void copy_fdt(void);
-extern void metag_dt_memblock_reserve(void);
 
 #endif /* __ASM_METAG_PROM_H */