common: board_r: move init_addr_map() to init.h

asm/mmu.h include is currently guarded by CONFIG_ADDR_MAP ifdef because
the header is only present on arm and powerpc. In order to remove the
dependency on this header and the associated ifdef, move init_addr_map()
declaration to init.h, since it is only called during the common init
sequence.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>