ArmPlatformPkg/NorFlashDxe: use one GUID plus index to identify flash banks

Currently, each flash bank controlled by ArmPlatformPkg/NorFlashDxe
has its own VendorHw GUID, and instances of NorFlashPlatformLib
describe each bank to the driver, along with the GUID for each.

This works ok for bare metal platforms, but it would be useful for
virtual platforms if we could obtain this information from a
device tree, which would require us to invent GUIDs on the fly,
given that the 'cfi-flash' binding does not include a GUID.

So instead, let's switch to a single GUID for all flash banks,
and update the driver's device path handling to include an index
to identify each bank uniquely.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Thomas Abraham <thomas.abraham@arm.com>