Fix MISRA Rule 5.3 Part 5

Use a _ prefix for macro arguments to prevent that argument from
hiding variables of the same name in the outer scope

Rule 5.3: An identifier declared in an inner scope shall not
          hide an identifier declared in an outer scope

Fixed For:
    make LOG_LEVEL=50 PLAT=juno

Change-Id: I575fbc96e8267f2b075e88def1f6e3185394613a
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>