dt-bindings: gpio: cavium,thunder-8890: Remove DT binding

Remove the cavium,thunder-8890 GPIO binding as there are no active
use cases. The binding is unused as the corresponding kernel driver
binds via PCI and not the compatible.

Signed-off-by: Shi Hao <[email protected]>
Reviewed-by: Krzysztof Kozlowski <[email protected]>
Link: https://patch.msgid.link/[email protected]
[Bartosz: tweaked the commit message]
Signed-off-by: Bartosz Golaszewski <[email protected]>

Author: i-shihao