ARM: 9219/1: fix undeclared soft_restart

The soft_restart() is declared in <asm/system_misc.h> so
include that to fix the following sparse warning:

arch/arm/kernel/reboot.c:78:6: warning: symbol 'soft_restart' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>