bcm2835-i2s: Enable MMAP support via a DT property

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

RPi commit 7ee829fd ("bcm2708-i2s:
Enable MMAP support via a DT property and overlay")

The i2s driver used to claim to support MMAP, but that feature was disabled
when some problems were found. Add the ability to enable this feature
through Device Tree, using the i2s-mmap overlay.

See: #1004

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