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 SPD=tspd

Change-Id: I2d711b9584c4cb9ba3814ecd2ca65a42b7e24179
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>