spi: bcm2835: Enable shared interrupt support

bcm2711, Rasberry Pi 4's SoC, shares one interrupt for multiple
instances of the bcm2835 SPI controller. So this enables shared
interrupt support for them.

The early bail out in the interrupt routine avoids messing with buffers
of transfers being done by other means. Otherwise, the driver can handle
receiving interrupts asserted by other controllers during an IRQ based
transfer.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20200528185805.28991-1-nsaenzjulienne@suse.de


Signed-off-by: Mark Brown <broonie@kernel.org>