RISC-V: Move setup_bootmem() to mm/init.c

The setup_bootmem() mainly populates memblocks and does early memory
reservations. The right location for this function is mm/init.c. It
calls setup_initrd() so we move that as well.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>