spi: bcm63xx-spi: allow building for BMIPS

bcm63xx-spi controller is present on several BMIPS SoCs (BCM6358, BCM6362,
BCM6368 and BCM63268).

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20200615090943.2936839-3-noltari@gmail.com


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