xilinx: versal: Setup 30MHz as default spi frequency

Align default SPI configuration with ZynqMP/Versal NET.
There is no reason to run on lower frequencies.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/c1d6ebd659f3002649b1200c926f8b9ed3132085.1698749448.git.michal.simek@amd.com