armv8/fsl-lsch3: Convert flushing L3 to assembly to avoid using stack

Flushing L3 cache in CCN-504 requries d-cache to be disabled. Using
assembly function to guarantee stack is not used before flushing is
completed. Timeout is needed for simualtor on which CCN-504 is not
implemented. Return value can be checked for timeout situation.

Change bootm.c to disable dcache instead of simply flushing, required
by flushing L3.

Signed-off-by: York Sun <yorksun@freescale.com>