s390/diag324: Retrieve power readings via diag 0x324

s390/diag324: Retrieve power readings via diag 0x324

Retrieve electrical power readings for resources in a computing
environment via diag 0x324. diag 0x324 stores the power readings in the
power information block (pib).

Provide power readings from pib via diag324 ioctl interface.  diag324
ioctl provides new pib to the user only if the threshold time has passed
since the last call. Otherwise, cache data is returned. When there are
no active readers, cleanup of pib buffer is performed.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>