spl: fit: Factor out FIT parsing and use a context struct

The logical steps in spl_load_simple_fit() are difficult to follow.
I think the long comments, ifdefs, and ungodly number of variables
seriously affect the readability. In particular, it violates section 6
of the coding style, paragraphs (3), and (4).

The purpose of this patch is to improve the situation by
  - Factoring out initialization and parsing to separate functions
  - Reduce the number of variables by using a context structure
This change introduces no functional changes.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>