zynq: Use arch_cpu_init() instead of lowlevel_init()

Zynq lowlevel_init() was implemented in C but stack
pointer is setup after function call in _main().
Move architecture setup to arch_cpu_init() which is call
as the first function in board_init_f() which
already have correct stack pointer.

Reported-by: Sven Schwermer <sven.schwermer@tuhh.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>