m68k: switch to MEMBLOCK + NO_BOOTMEM

In m68k the physical memory is described by [memory_start, memory_end] for
!MMU variant and by m68k_memory array of memory ranges for the MMU version.
This information is directly use to register the physical memory with
memblock.

The reserve_bootmem() calls are replaced with memblock_reserve() and the
bootmap bitmap allocation is simply dropped.

Since the MMU variant creates early mappings only for the small part of the
memory we force bottom-up allocations in memblock.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>