ASoC: dwc: Permit sample rates up to 384kHz

The BCM2835 I2S block advertises clock rates up to 384kHz, and there's
no reason why RP1's DWC I2S block shouldn't do the same.

See: https://github.com/raspberrypi/linux/issues/5748



Signed-off-by: Phil Elwell <phil@raspberrypi.com>