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 PLAT=fvp USE_COHERENT_MEM=0

Change-Id: If50c583d3b63799ee6852626b15be00c0f6b10a0
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>