ARM: dts: imx7: Unify temp-grade and speed-grade nodes

The following warning is seen when building with W=1:

arch/arm/boot/dts/imx7s.dtsi:551.39-553.7: Warning (unique_unit_address): /soc/aips-bus@30000000/ocotp-ctrl@30350000/temp-grade@10: duplicate unit-address (also used in node /soc/aips-bus@30000000/ocotp-ctrl@30350000/speed-grade@10)

Since temp-grade and speed-grade point to the same node, replace them by
a single one to avoid the duplicate unit-address warning.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>