s390/ctlreg: add local_ctl_load() and local_ctl_store()

s390/ctlreg: add local_ctl_load() and local_ctl_store()

Add local_ctl_load() and local_ctl_store() which load and store contents
for only a single control register.

This allows for easier to read code, but also better type checking,
since __local_ctl_load() and __local_ctl_store() do not come with any
type checking at all (which will be changed with subsequent patches).

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>