ARM: bcm283x: Enable DMA support for SPI controller

Without this, the driver for the BCM2835 SPI controller uses interrupt
mode instead of DMA mode, incurring a significant performance penalty.
The Foundation's device tree has had these attributes for years, but for
some reason they were never upstreamed.

They were originally contributed by Noralf Trønnes and Martin Sperl:
https://github.com/raspberrypi/linux/commit/25f3e064afc8
https://github.com/raspberrypi/linux/commit/e0edb52b47e6

The DREQ numbers 6 and 7 are documented in section 4.2.1.3 of:
https://www.raspberrypi.org/app/uploads/2012/02/BCM2835-ARM-Peripherals.pdf



Tested-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Cc: Martin Sperl <kernel@martin.sperl.org>
Cc: Noralf Trønnes <noralf@tronnes.org>