arm: dts: k3-am625-verdin-r5: Change CPU frequency to 800MHz

The lowest speed grade of Toradex AM62 SoMs is K speed, resulting in a
max value of 800MHz for the CPU operating frequency. A solution with
runtime selection of the CPU frequency is already planned to avoid these
kinds of problems in the future.

Fixes: 8fb8a6d4 ("arm: dts: k3-am625-verdin-r5:Change CPU frequency to 1000MHz")
Signed-off-by: Joao Paulo Goncalves <joao.goncalves@toradex.com>