ARM: dts: imx: Adjust CPU nodes

Make CPU nodes consistent throughout the i.MX dts files, which
also matches the description from ePAPR spec.

This also fixes the following W=1 warning in some cases:

Warning (unit_address_vs_reg): Node /cpus/cpu@0 has a unit name, but no reg property

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>