serial: ifx6x60: set SPI max_speed_hz based on platform type

Platforms containing the 6260 can run up to 25Mhz.

For these platforms set max_speed_hz to 25Mhz.

Signed-off-by: Russ Gorby <russ.gorby@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>