ppc/pnv: Begin a more complete ADU LPC model for POWER9/10

ppc/pnv: Begin a more complete ADU LPC model for POWER9/10

This implements a framework for an ADU unit model.

The ADU unit actually implements XSCOM, which is the bridge between MMIO
and PIB. However it also includes control and status registers and other
functions that are exposed as PIB (xscom) registers.

To keep things simple, pnv_xscom.c remains the XSCOM bridge
implementation, and pnv_adu.c implements the ADU registers and other
functions.

So far, just the ADU no-op registers in the pnv_xscom.c default handler
are moved over to the adu model.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>