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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/spi/brcm,bcm2835-spi.txt

···
 auxiliary block. This binding applies to the SPI0 controller.
 
 Required properties:
-- compatible: Should be "brcm,bcm2835-spi".
+- compatible: Should be one of "brcm,bcm2835-spi" for BCM2835/2836/2837 or
+  "brcm,bcm2711-spi" for BCM2711 or "brcm,bcm7211-spi" for BCM7211.
 - reg: Should contain register location and length.
 - interrupts: Should contain interrupt.
 - clocks: The clock feeding the SPI controller.