s390: Move sske_frame() to a header

s390: Move sske_frame() to a header

Move the sske_frame() function to asm/pgtable.h, so it can be used in
other modules too.

Opportunistically convert the .insn opcode specification to the
appropriate mnemonic.

Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Reviewed-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Reviewed-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>