s390/Kconfig: add z13s and z14 ZR1 to TUNE descriptions

The names for the z13s and z14 ZR1 machines are missing for the
TUNE_Z13 and TUNE_Z14 descriptions. Just add them.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>