ArmPlatformPkg/NorFlashDxe: prepare for devicepath format change

ArmPlatformPkg/NorFlashDxe: prepare for devicepath format change

A subsequent patch will change the layout of devicepath nodes
produced by this driver. In preparation, make some tweaks to
the code to use a packed struct for the devicepath and to pass
the device index to NorFlashCreateInstance(). These are cosmetic
changes only, the resulting binaries should be identical.

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>