omap: beagle_x15: add MAINTAINERS

Commit 1e4ad74b (beagle_x15: add board support for Beagle x15)
missed to add board/ti/beagle_x15/MAINTAINERS.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Felipe Balbi <balbi@ti.com>