dt-bindings: spi: Document bcm2711 and bcm7211 SPI compatible

The BCM2711 and BCM7211 chips use the BCM2835 SPI controller, but there
are severl instances of those in the system and they all share the same
interrupt line. Document specific compatible strings such that the
driver can take appropriate actions.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20200604034655.15930-2-f.fainelli@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>