Fix broken SMP startup on Armada XP after r265694

During Armada's platform_mp_start_ap(), mptramp code
is being copied to the specific physical location (0xffff0000).
Before r265694 the address to which the code should be copied
was equal to the address of mpentry routine that followed the
mptramp in locore.S. Now the mptramp end address should be
exported and used as a copy limit.

Reviewed by: gber
Avoid using hard-coded SoC's register address in mptramp code for Armada XP

SoC's registers base address may differ between boards
(0xf1000000 or 0xd0000000). Therefore, in order to use
the proper CPU Boot Address Redirect register during SMP
initialization in mptramp the real, physical address has
to be passed to mptramp based on the value from DT.

Reviewed by: gber