BeagleBoneBlackPkg: Configure Stack Base Address

Set PcdCPUCoresStackBase according to AM335x Memory Map (TRM 26.1.3.2) for
CreatePpiList() to work. Use correct ArmPlatformLib.h.
BeagleBoneBlackPkg: Add ArmPlatformPkg/PrePeiCore

Add Pre-PEI Core that is called by the SPL on boot. SEC is no longer needed.
BeagleBoneBlackPkg: Use FixedPcdGet in PrePeiCore

Use FixedPcdGet32() and disable redundant calls.