ARM: zynq: Enable the Neon instructions

Added the lowlevel_init to enable the Neon instructions.

Initially the u-boot was causing undefined instruction
exception if loaded through tcl, and working fine if loaded
through FSBL. The exception was causing in convertion formula
of given time to ticks. It was because, the Neon instructions
were disabled and hence causing the undefined exception. In
FSBL case, the FSBL was enabling the Neon instructions. Hence,
added the lowlevel_init to enable the Neon instructions.

Also enable neon instructions for non-xilinx toolchain.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Acked-by: Radhey Shyam Pandey <radheys@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>