ppc/pnv: Add initial P9/10 SBE model

The SBE (Self Boot Engine) are on-chip microcontrollers that perform
early boot steps, as well as provide some runtime facilities (e.g.,
timer, secure register access, MPIPL). The latter facilities are
accessed mostly via a message system called SBEFIFO.

This driver provides initial emulation for the SBE runtime registers
and a very basic SBEFIFO implementation that provides the timer
command. This covers the basic SBE behaviour expected by skiboot when
booting.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20220811093726.1442343-1-npiggin@gmail.com>
[danielhb: fixed SBE_HOST_RESPONSE_MASK long line]
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

hw/ppc/meson.build

@@ -46,6 +46,7 @@ ppc_ss.add(when: 'CONFIG_POWERNV', if_true: files(
   'pnv_lpc.c',
   'pnv_psi.c',
   'pnv_occ.c',
+  'pnv_sbe.c',
   'pnv_bmc.c',
   'pnv_homer.c',
   'pnv_pnor.c',