ARM: imx: Add MXC_CPU_IMX6ULL and cpu_is_imx6ull

Support for imx6ull is already present but it's based on
of_machine_is_compatible("fsl,imx6ull") checks. Add it to the MXC_CPU_*
enumeration as well.

This also fixes /sys/devices/soc0/soc_id reading "Unknown".

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