docs/system: Document the removal of "compat" property for POWER CPUs

docs/system: Document the removal of "compat" property for POWER CPUs

This is just an oversight.

Fixes: f518be3aa35b ("target/ppc: Remove "compat" property of server class POWER CPUs")
Cc: groug@kaod.org
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <161399328834.51902.14269239378658110394.stgit@bahia.lan>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
(cherry picked from commit a058b895079348d0854a027a42ce3396a4a00bb7)
Signed-off-by: Michael Roth <michael.roth@amd.com>