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The Arm SGI platforms can switch to using SCMI. So enable support for
SCMI and remove portions of code that would be unused after switching
to SCMI.

Change-Id: Ifd9e1c944745f703da5f970b5daf1be2b07ed14e
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>