arm: dts: k3-am625-verdin-r5: Change CPU frequency to 1000MHz

The same U-Boot binary is compatible with multiple Verdin AM62 board
variants. However, some of the SoC models can only operate at a maximum
speed of 1 GHz.

Previously, the boards with lower-speed grades were running at
overclocked speeds, leading to kernel complaints about unsupported
configurations.

To resolve this issue, the operating speed has been decreased to the
maximum allowable value across all Verdin AM62 board variants. As a
result, there is a regression in overall boot time, increasing by around
200 milliseconds for the faster SoC variant.

Signed-off-by: Vitor Soares <vitor.soares@toradex.com>
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>