Marvell/Drivers: Pp2Dxe: Fix cleaning of the BM pools

Hitherto mechanism of using static flag indicating
Buffer Manager disabled state was wrong and resulted
in cleaning only one controller's pools. This caused
bad page state when OS booted and attempted to
use the buffers.

This patch implements a fix, ensuring all controllers
will clean the Buffer Manager during ExitBootServices.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>