hw/arm/aspeed: Unlock SCU when running kernel

The ASPEED hardware contains a lock register for the SCU that disables
any writes to the SCU when it is locked. The machine comes up with the
lock enabled, but on all known hardware u-boot will unlock it and leave
it unlocked when loading the kernel.

This means the kernel expects the SCU to be unlocked. When booting from
an emulated ROM the normal u-boot unlock path is executed. Things don't
go well when booting using the -kernel command line, as u-boot does not
run first.

Change behaviour so that when a kernel is passed to the machine, set the
reset value of the SCU to be unlocked.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20171114122018.12204-1-joel@jms.id.au
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>