dt-bindings: gpmc: Correct location of generic gpmc binding

The binding bus/ti-gpmc.txt has been moved to
memory-controllers/omap-gpmc.txt.  Update all references to this in
order to make reading and understanding a given binding easier.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc:Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>