MAINTAINERS: merge TRIGGER SOURCE sections

MAINTAINERS: merge TRIGGER SOURCE sections

Merge all of Documentation/devicetree/bindings/trigger-source/* as a
single entry.

In the previous merge cycle, there was a merge conflict between the IIO
and SPI trees and we ended up with the current state, but the intention
was to have a single entry for all trigger-source bindings.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250815-spi-offload-trigger-followup-v1-1-8ec5413a8383@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>