plat: xilinx: versal: Remove cortex-a53 compilation
plat: xilinx: versal: Remove cortex-a53 compilation

Versal is a72 based that's why there is no reason to build low level
assemble code for a53.

Signed-off-by: Michal Simek <b20e59da6aeb5cea6c82b021069d69f63616c55d@xilinx.com>
Change-Id: Iff9cf2582102d951825b87fd9af18e831ca717d6

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware