MAINTAINERS: Add Vinod Koul as Generic PHY co-maintainer

Add Vinod Koul as Generic PHY Subsystem co-maintainer and move
the linux-phy to a shared repository.

Cc: Vinod Koul <vkoul@kernel.org>
Acked-By: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>