MAINTAINERS: Add staging branch for HyperBus

Update HyperBus entry with branch used to stage patches under
mtd.git. Also, add mailing list and patchwork queue information.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200227051212.15496-1-vigneshr@ti.com