plat/arm: Move norflash driver to drivers/ folder

This way it can be reused by other platforms if needed.

Note that this driver is designed to work with the Versatile Express NOR
flash of Juno and FVP. In said platforms, the memory is organized as an
interleaved memory of two chips with a 16 bit word.

Any platform that wishes to reuse it with a different configuration will
need to modify the driver so that it is more generic.

Change-Id: Ic721758425864e0cf42b7b9b04bf0d9513b6022e
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>