Platform/ARM: FVP: Fix variable usage in GetArmNameSpaceObject()

Building with Clang showed that `RootComplexCount` and
`DeviceIdMappingArrayCount` were unused. Use the
variables where they were intended to be used.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>