feat(fvp): measure critical data

Implemented a platform function 'plat_mboot_measure_critical_data' to
measure critical data and record its measurement using the Event Log
driver.
'bl2_plat_mboot_finish' function invokes this platform function
immediately after populating the critical data.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: Ia198295c6e07ab26d436eab1ff90df2cf28303af