ARM: 8360/1: add secondary_startup_arm prototype in header file

Put secondary_startup_arm() prototype in arch/arm/include/asm/smp.h
so users doesn't have to add extern prototype in their code.

Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>