Hi…SPI flash uses standard SPI mode 0 or 3, not I2S (a protocol for audio chips). The device is very fast and can probably use any available ARM SPI clock, in case of doubt use moderate clock frequency, e.g. 10 – 20 MHz.
You have 50 ohm SCK parallel termination in your schematic which can hardly be driven by the ARM clock output. If you expect reflections with longer PCB traces, place a series termination (e.g. 50 ohm) near the CPU and no termination at the flash side.