ARM: dts: imx6qdl-tqma6: fix LM75 compatible string

Specify the National LM75 sensor including its vendor name, as mandated
by the binding docs.

Fixes: cac849e9

 ("ARM: dts: imx6qdl: add TQMa6{S,Q,QP} SoM")
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>