Sub-CRQs are registred with firmware via a hypercall. Abstract that
interface into a simpler helper function.

Link: https://lore.kernel.org/r/20210114203148.246656-6-tyreld@linux.ibm.comReviewed-by: NBrian King <brking@linux.vnet.ibm.com>
Signed-off-by: NTyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>