dma: Add possibility for up to 3 DMA Controllers

Currently only 1 is allowed.  While we're at it, let's generify the
existing entries, since they could be used by all controllers, not
just QMSI.

Change-Id: Iec5d195fff239931b21a7584eb4b642b40f95be5
Signed-off-by: Lee Jones <lee.jones@linaro.org>