ARM: dts: imx6ull-dhcor: Remove 900MHz operating point

Due to CPU lifetime consideration of the SoC manufacturer [1] and
the preferred area of operation in the industrial related
environment, set the maximum frequency for each DHCOM i.MX6ULL to
792MHz, as with the industrial type. This is done by removing the
operating point at 900MHz and set the clock-frequency to 792000000.

[1] https://www.nxp.com/docs/en/application-note/AN5337.pdf



Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>