aspeed: sbc: Allow per-machine settings

aspeed: sbc: Allow per-machine settings

In order to correctly report secure boot running firmware the values
of certain registers must be set.

We don't yet have documentation from ASPEED on what they mean. The
meaning is inferred from u-boot's use of them.

Introduce properties so the settings can be configured per-machine.

Reviewed-by: Peter Delevoryas <pdel@fb.com>
Tested-by: Peter Delevoryas <pdel@fb.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Message-Id: <20220628154740.1117349-4-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>