Platform/ARM: import ARM platform specific BdsLib header

The BdsLib library has been moved into Platform/ARM a while ago,
but the BdsLib.h header was left behind, and so all users in
Platform/ARM are still relying on it to be available in ArmPkg.

So let's add a copy to Platform/ARM and wire it up, so we can
drop it from ArmPkg going forward. Note that the BdsLib
implementation included ArmLib.h from ArmPkg without using
anything it provides, so drop that false dependency as well.

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