i.MX 8M Nano has four Cortex-A CPUs, not six.  Using higher value is
harmless but adjust it to match real HW.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>