packagegroup-fsl-tools-benchmark: Add cpuburn-neon back

cpuburn-neon had been commented out in the previously
available packagegroup-fsl recipe, due to a source code
issue that prevented build in denzil.

When packagegroup-fsl recipe was split, cpubrun-neon package
was not included in any of the new packagegroups.

This patch adds this tool back, as its source code has been
fixed and is now buildable, into the tools-benchmark packagegroup.

The cpubrun-neon package inclusion is actually conditional,
depending on the presence of the 'neon' flag in the TUNE_FEATURES
variable. For this reason, the packagegroup ARCH needs to be
MACHINE specific now.

Signed-off-by: Rogerio Nunes <rogerio.nunes@freescale.com>