board: amlogic: Rename folder for Amlogic boards

s/hardkernel/amlogic/ to have a single place for all the amlogic-based
boards.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Carlo Caione <carlo@endlessm.com>
Acked-by: Beniamino Galvani <b.galvani@gmail.com>