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qemu: introducing sub-platforms to qemu platform

This commit change the plat/qemu directory structure into:

`-- plat
    `-- qemu
        |-- common    (files shared with all qemu subplatforms)
        |-- qemu      (original qemu platform)
        |-- qemu_sbsa (new sqemu_sbsa platform)
        |-- subplat1
        `-- subplat2

This opens the possibility of adding new qemu sub-platforms which reuse
existing common platform code. The first platform which will leverage new
structure will be SBSA platform.
Signed-off-by: Radoslaw Biernacki <radoslaw.biernacki@linaro.org>
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: Id0d8133e1fffc1b574b69aa2770ebc02bb837a9b