SbsaQemu: add port for SbsaQemu platform

The QEMU sbsa-ref platform provides an SBSA-compliant platform,
providing EL3/EL2 support, non-virtio interfaces, etc. This patch adds
initial support for this platform.

We are using FDF to compose EFI flash images with TF-A images.
Flash0 (secure) is used by BL1 and FIP (BL2 + BL31).
Flash1 contains EFI code and EFI variables.

Signed-off-by: Tanmay Jagdale <tanmay.jagdale@linaro.org>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>