thermal: imx: Add support for reading OCOTP through nvmem

On newer imx SOCs accessing OCOTP directly is wrong because the ocotp
clock needs to be enabled first. Add support for reading those same
values through the nvmem API instead.

The older path is preserved for compatibility with older dts and because
it works correctly on imx6qdl chips.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>