ARM: integrator: move flash registration to device tree

The flash on the Integrator was already defined by the device
tree, but VPP control and flash protection was in the
boardfiles. Simply add the compatible string
"arm,versatile-flash" and the special add-on code for flash
programming voltage and protection kicks in in the MTD
layer.

Remove the board file code and augment the device tree in
one go for seamless transition.

Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>