ARM: 8581/1: add missing <asm/prom.h> to arch/arm/kernel/devtree.c

Fix the following warnings by including declarations
from <asm/prom.h>:

arch/arm/kernel/devtree.c:69:13: warning: symbol 'arm_dt_init_cpu_maps' was not declared. Should it be static?
arch/arm/kernel/devtree.c:210:27: warning: symbol 'setup_machine_fdt' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>