m68k: move ColdFire _bootmem_alloc code

The early ColdFire bootmem_alloc() code is currently only included in
the board support for the Coldire 54xx platforms. It will be used on all
ColdFire MMU enabled platforms as others are supported. So move the
mcf54xx_bootmem_alloc() function to be generally available to all MMU
enabled ColdFire parts (and use a more generic name for it).

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>