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>

Documentation/devicetree/bindings/mtd/arm-versatile.txt

 Flash device on ARM Versatile board
 
+These flash chips are found in the ARM reference designs like Integrator,
+Versatile, RealView, Versatile Express etc.
+
+They are regular CFI compatible (Intel or AMD extended) flash chips with
+some special write protect/VPP bits that can be controlled by the machine's
+system controller.
+
 Required properties:
-- compatible : must be "arm,versatile-flash";
+- compatible : must be "arm,versatile-flash", "cfi-flash";
+- reg : memory address for the flash chip
 - bank-width : width in bytes of flash interface.
 
+For the rest of the properties, see mtd-physmap.txt.
+
 The device tree may optionally contain sub-nodes describing partitions of the
 address space. See partition.txt for more detail.
+
+Example:
+
+flash@34000000 {
+	compatible = "arm,versatile-flash", "cfi-flash";
+	reg = <0x34000000 0x4000000>;
+	bank-width = <4>;
+};