arm64: dts: ti: k3-j721e-som-p0: Set OSPI frequency to 25MHz

This is the frequency that the flash has been successfully tested with
in Octal mode (without PHY). Use this frequency now that we have
introduced Octal mode support. The read delay is also changed to
correspond the frequency change.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>