mtd: augment the "arm,versatile-flash" bindings

The bindings for the "arm,versatile-flash" device was merged in
commit 3ba7222a
"arm/versatile: Add device tree support" but was never used
for anything.

Versatile flash chips are actually just standard CFI chips,
but they have one or two bits in a system controller to control
VPP and write protection. Let's use this compatible string in
conjunction with "cfi-flash" to indicate that we have a
normal CFI flash with some extra Versatile-specific protection.

Cc: devicetree@vger.kernel.org
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>