We've a number of C functions with external linkage that are only called
from assembly. -Wmissing-prototypes warns about these unless they have
a prototype that goes unused. Let's standardize on using the asmlinkage
'storage class' to mark such declarations.

As <linux/linkage.h> defines asmlinkage to naught unless <asm/linkage.h>
does, it's sufficient to add an empty header to make this usable on x86
as well.

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>