powerpc: Document details on H_SCM_HEALTH hcall

Add documentation to 'papr_hcalls.rst' describing the bitmap flags
that are returned from H_SCM_HEALTH hcall as per the PAPR-SCM
specification.

Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Acked-by: Ira Weiny <ira.weiny@intel.com>
Cc: "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20200615124407.32596-2-vaibhav@linux.ibm.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>