ARM: dts: imx6sx: Use nvmem-cells for tempmon

ARM: dts: imx6sx: Use nvmem-cells for tempmon

On imx6sx accessing OCOTP directly is wrong because the ocotp clock
needs to be enabled first. Use the nvmem-cells binding instead.

This requirement does not apply to older imx6qdl chips because there the
ocotp access clock (clk_ipg_s) is always enabled.

This is visible by comparing the "System Clocks, Gating, and Override"
tables (OCOTP rows) in the 6DQ and 6SX manuals:
http://www.nxp.com/assets/documents/data/en/reference-manuals/IMX6SXRM.pdf
http://www.nxp.com/assets/documents/data/en/reference-manuals/IMX6DQRM.pdf

This happens to work right now because the ocotp clock might be enabled
for some other reason. In particular the it might be enabled from the
bootloader and it only gets disabled late during boot in
clk_disable_unused, after imx-thermal has completed probing.

If imx-thermal is compiled as a module then the system can hang on
probe.

Reported-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>