Marvell/Armada7k8k: Refactor reserving memory regions

Extract reserving memory region in the Hob list
into a separate routine. It is a preparation for adding
multiple of such regions in a following patch.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>