binman: Add support for Intel FIT

A Firmware Image Table (FIT) is a data structure defined by Intel which
contains information about various things needed by the SoC, such as
microcode.

Add support for this entry as well as the pointer to it. The contents of
FIT are fixed at present. Future work is needed to support adding
microcode, etc.

Signed-off-by: Simon Glass <sjg@chromium.org>