bcm2835-i2s: Register PCM device

Code ported from bcm2708-i2s driver in Raspberry Pi tree.

RPi commit ba46b493 ("ASoC: Add
support for BCM2708")

This driver adds support for digital audio (I2S)
for the BCM2708 SoC that is used by the
Raspberry Pi. External audio codecs can be
connected to the Raspberry Pi via P5 header.

It relies on cyclic DMA engine support for BCM2708.

Signed-off-by: Florian Meier <florian.meier@koalo.de>

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