mtd: afs: factor the IIS read into partition parser

Factor the IIS (Image Information Structure) reading into the
partition parser, giving us a single, clean partition parser
function.

Cc: Ryan Harkin <ryan.harkin@linaro.org>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Richard Weinberger <richard@nod.at>