MAINTAINERS: Add GPIO Aggregator section

Add a maintainership section for the GPIO Aggregator, covering
documentation and driver source code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Tested-by: Eugeniu Rosca <erosca@de.adit-jv.com>
---
v7:
  - Maintained => Supported,

v6:
  - No changes,

v5:
  - Add Reviewed-by, Tested-by,

v4:
  - Drop controversial GPIO repeater,

v3:
  - New.
1 file changed