arm64: dts: ti: am654-base-board: Enable 8D-8D-8D mode on OSPI

arm64: dts: ti: am654-base-board: Enable 8D-8D-8D mode on OSPI

Set the Tx bus width to 8 so 8D-8D-8D mode can be selected. Change the
frequency to 25 MHz. This is the frequency that the flash has been
successfully tested with in Octal DTR mode. The total performance should
still increase since 8D-8D-8D mode should be at least twice as fast as
1S-1S-8S mode.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20210305153926.3479-3-p.yadav@ti.com