dmaengine: bcm2835: Load driver early and support legacy API

Load driver early since at least bcm2708_fb doesn't support deferred
probing and even if it did, we don't want the video driver deferred.

Support the legacy DMA API which is needed by bcm2708_fb
(but only using the dedicated dma channel 0).

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>