powerpc: Provide initial documentation for PAPR hcalls

powerpc: Provide initial documentation for PAPR hcalls

This doc patch provides an initial description of the hcall op-codes
that are used by Linux kernel running as a guest (LPAR) on top of
PowerVM or any other sPAPR compliant hyper-visor (e.g qemu).

Apart from documenting the hcalls the doc-patch also provides a
rudimentary overview of how hcall ABI, how they are issued with the
Linux kernel and how information/control flows between the guest and
hypervisor.

Signed-off-by: Vaibhav Jain <[email protected]>
Reviewed-by: Laurent Dufour <[email protected]>
Acked-by: Nicholas Piggin <[email protected]>
[mpe: Add SPDX tag, add it to index.rst]
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]