Calling arm_cpu_lowlevel_init() from zynq_cpu_lowlevel_init() adds a stack
push/pop to the latter function which doesn't work this early in the boot.

As the BootROM apparently hands proccessor control to us in abort(!?!)
mode, setting up a stack requires duplicating most of arm_cpu_lowlevel_init().
To get around this catch-22 move the call to arm_cpu_lowlevel_init back into
the board lowlevel start function, so we don't need a stack at all.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>