docs: add details about plat_mboot_measure_key function

Added details of 'plat_mboot_measure_key' function in the porting-guide.

Change-Id: Id62211abc0ba13a0f581dc8e24c7b367afe2dcf5
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
1 file changed