ASoC: bcm2835: Support additional samplerates up to 384kHz

Sample rates are only restricted by the capabilities of the
clock driver, so use SNDRV_PCM_RATE_CONTINUOUS instead of
SNDRV_PCM_RATE_8000_192000.

Tests (eg with pcm5122) have shown that bcm2835 works fine
in 384kHz/32bit stereo mode, so change the maximum allowed
rate from 192kHz to 384kHz.

Signed-off-by: Matthias Reichl <hias@horus.com>