plat/sgi: switch to using scmi

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>
4 files changed