hexagon: switch over to generic free_initmem()

hexagon implementation of free_initmem() is currently empty and marked
with comment

 * Todo:  free pages between __init_begin and __init_end; possibly
 * some devtree related stuff as well.

Switch it to the generic implementation.

Link: http://lkml.kernel.org/r/1550515285-17446-3-git-send-email-rppt@linux.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Richard Kuo <rkuo@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>