arm64: dts: ti: k3-am625-sk: Add 1.4GHz OPP

The 1.4 GHz OPP requires supported silicon variant (T speed grade) and
also VDD_CORE to be at 0.85V.  All production revisions of the AM625-SK
have both so we can enable the 1.4 GHz OPP for it.  Any other boards
based on this design should verify that they have the right silicon
variant and the right power tree before adding 1.4 GHz support in their
board dts file.

Signed-off-by: Vibhore Vardhan <vibhore@ti.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>