alpha: fix hang caused by the bootmem removal

The conversion of alpha to memblock as the early memory manager caused
boot to hang as described at [1].

The issue is caused because for CONFIG_DISCTONTIGMEM=y case,
memblock_add() is called using memory start PFN that had been rounded
down to the nearest 8Mb and it caused memblock to see more memory that
is actually present in the system.

Besides, memblock allocates memory from high addresses while bootmem was
using low memory, which broke the assumption that early allocations are
always accessible by the hardware.

This patch ensures that memblock_add() is using the correct PFN for the
memory start and forces memblock to use bottom-up allocations.

[1] https://lkml.org/lkml/2018/11/22/1032

Link: http://lkml.kernel.org/r/1543233216-25833-1-git-send-email-rppt@linux.ibm.com
Reported-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Tested-by: Meelis Roos <mroos@linux.ee>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>