parisc: use memblock_alloc() instead of custom get_memblock()

The get_memblock() function implements custom bottom-up memblock allocator.
Setting 'memblock_bottom_up = true' before any memblock allocation is done
allows replacing get_memblock() calls with memblock_alloc().

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Tested-by: Helge Deller <deller@gmx.de>
Signed-off-by: Helge Deller <deller@gmx.de>