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 <i.shihao.999@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260408093313.17025-1-i.shihao.999@gmail.com
[Bartosz: tweaked the commit message]
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>

Author: i-shihao