lib: s390x: add functions to set and clear PSW bits

Add some functions to set and/or clear bits in the PSW.

Also introduce PSW_MASK_KEY and re-order the PSW_MASK_* constants so
they are descending in value.

This should improve code readability.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>