spl: fit: Drop 'length' argument to board_spl_fit_post_load()

The size is derived from the FIT image itself. Any alignment
requirements are machine-specific and known by the board code. Thus
the total length can be derived from the FIT image and knowledge of
the platform. The 'length' argument is redundant. Remove it.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
CC: Matt Porter <mporter@konsulko.com>