ArmPlatformPkg: Fix Ecc error 8005

ArmPlatformPkg: Fix Ecc error 8005

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'

Indeed, according to the EDK II C Coding Standards
Specification, s5.6.2.2 "Enumerated Types" and
s4.3.4 Function and Data Names, elements of an
enumerated type shoud be a mixed upper- and
lower-case text.

A max element is also added, as advised by
s5.6.2.2.3 of the same document.

Reference:
https://edk2-docs.gitbook.io/edk-ii-c-coding-standards-specification/

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>