When the SSC acts as BCK master, use a ratnum rule to limit
the rate instead of only doing the standard rates. When the SSC
acts as BCK slave, allow any BCK frequency up to the SSC master
clock, divided by either of 2, 3 or 6.

Put a cap at 384kHz. Who's /ever/ going to need more than that?

The divider of 2, 3 or 6 is selected based on the Serial Clock Ratio
Considerations section from the SSC documentation:

    The Transmitter and the Receiver can be programmed to operate
    with the clock signals provided on either the TK or RK pins.
    This allows the SSC to support many slave-mode data transfers.
    In this case, the maximum clock speed allowed on the RK pin is:
    - Peripheral clock divided by 2 if Receiver Frame Synchro is input
    - Peripheral clock divided by 3 if Receiver Frame Synchro is output
    In addition, the maximum clock speed allowed on the TK pin is:
    - Peripheral clock divided by 6 if Transmit Frame Synchro is input
    - Peripheral clock divided by 2 if Transmit Frame Synchro is output
Signed-off-by: Peter Rosin <peda@axentia.se>
Acked-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>