board: qualcomm: add a MAINTAINERS file

board: qualcomm: add a MAINTAINERS file

Add a MAINTAINERS file for assigning maintainership of a particular
Qualcomm platform or board (if board specific files exist). It is
intended to supplement the treewide ARM SNAPDRAGON maintainers with
folks who have a more specific interest.

Add an initial entry for the QCS6490 platform which powers the RB3 Gen 2 board.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250604113452.1075503-2-casey.connolly@linaro.org
Signed-off-by: Casey Connolly <casey.connolly@linaro.org>