docs(measured-boot): add a platform function for critical data

Added a platform function to measure the critical data and record its
measurement.
Also, corrected a return value of 'plat_mboot_measure_image' function
in the documentation.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I576676f654e517c2010ca1d5a87a1f7277d581c3