dt-bindings: mtd: elm: Correct compatible string requirement

The binding says that the compatible string must be "ti,am33xx-elm"
but the code checks only for, and all functioning users set, this as
"ti,am3352-elm" so correct the binding.

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>