hw/misc/sbsa_ec : Add an embedded controller for sbsa-ref

hw/misc/sbsa_ec : Add an embedded controller for sbsa-ref

A difference between sbsa platform and the virt platform is PSCI is
handled by ARM-TF in the sbsa platform. This means that the PSCI code
there needs to communicate some of the platform power changes down
to the qemu code for things like shutdown/reset control.

Space has been left to extend the EC if we find other use cases in
future where ARM-TF and qemu need to communicate.

Signed-off-by: Graeme Gregory <graeme@nuviainc.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>
Tested-by: Leif Lindholm <leif@nuviainc.com>
Message-id: 20200826141952.136164-2-graeme@nuviainc.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>