imx6: imx: thermal: use CPU temperature grade info for thresholds

This is a backport of a2291badc355d58ead5c19ae0609468947416040 from thermal-soc
accepted upstream.

The IMX6Q/IMX6DL SoC's have a 2-bit temperature grade stored in OTP. Instead
of assuming 85C for passive cooling threshold and 100C for critical base
these thresholds off the thermal gade max CPU temperature:
 - passive threshold: max - 10C
 - critical threshold: max - 5C

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48011 3c298f89-4303-0410-b956-a3cf2f4a3e73