dt/bindings: bcm2835: add interrupt-names property

Added standard interrupt-names property so that
platform_get_irq_byname() can get used to fetch the
interrupt corresponding to each dma_channel
instead of the current platform_get_irq() with
an assumed ordering of the interrupts.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Eric Anholt <eric@anholt.net>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>