s390/cio: add tpi.h header file

For future work we need the struct tpi_info declaration in asm/ptrace.h.
Due to circular dependencies it cannot stay in asm/lowcore.h or asm/cio.h,
which would be the preferred location. Therefore add it in its own header
file.

Also fix a typo in the length of a reserved field that did not have a
functional effect beyond an incorrect field value in the s390_cio_tpi
tracepoint.

Fixes: 2ab59de7 ("s390/cio: Consolidate inline assemblies and related data definitions")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>