fix(arm): use PLAT instead of TARGET_PLATFORM

There might be several platforms which use the
TARGET_PLATFORM build option to differentiate the code
between the platform variants.

Use of TARGET_PLATFORM in the common code leads to build
failures instead use PLAT build option.

Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>
Change-Id: I9724caf875bd56225e035ecffa8b9ca1a50d3401
1 file changed