dt-bindings: mfd: syscon: Add amd,pensando-elba-syscon compatible

Add the AMD Pensando Elba SoC system registers compatible

Signed-off-by: Brad Larson <blarson@amd.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230119035136.21603-6-blarson@amd.com