ArmPkg: Fix Ecc error 8005 for SCMI_MESSAGE_ID

ArmPkg: Fix Ecc error 8005 for SCMI_MESSAGE_ID

This patch fixes the following Ecc reported error:
Variable name does not follow the rules:
1. First character should be upper case
2. Must contain lower case characters
3. No white space characters
4. Global variable name must start with a 'g'

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>