ARM: 9307/1: nommu: include asm/idmap.h

setup_mm_for_reboot() is declared in asm/idmap.h but that is not included
for the definition, causing a W=1 warning:

arch/arm/mm/nommu.c:178:6: error: no previous prototype for 'setup_mm_for_reboot' [-Werror=missing-prototypes]

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>