readme: Update SCMI reference to v3.1

Created by: chuyue-luo-arm

Update readme.md so that the reference to SCMI points to v3.1 instead of v1.0.

Signed-off-by: Chuyue Luo Chuyue.Luo@arm.com Change-Id: I85d04d53b1da3c45f58c15f1524c6e9e4ad07a03