dt-bindings: interrupt-controller: Merge BCM3380 with BCM7120

The two bindings are very similar and should be covered by the same
document, do that so we can get rid of an additional binding file.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211217160546.497012-3-f.fainelli@gmail.com