Marvell/Library: RealTimeClockLib: Simplify obtaining base address

Hitherto mechanism of obtaining RTC base address proved
to be not flexible enough to support more than one SoC
family. Because there can be a single controller in use
anyway, this patch drops utilization of MvHwDescLib
header with hardcoded structure and replace it with
simple UINT64 PCD.

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