refactor(measured-boot): split out a few Event Log driver functions

refactor(measured-boot): split out a few Event Log driver functions

Reorganized a few Event Log functions into multiple functions so that
they can be used for the upcoming DRTM feature. This change mainly
implements below new functions -
1. event_log_buf_init - called by 'event_log_init' to initialise Event
   Log buffer
2. event_log_write_specid_event - called by 'event_log_fixed_header' to
   write specification id event to Event Log buffer
3. event_log_measure and event_log_record - called by
   'event_log_measure_and_record' to measure and record the measurement
   to the Event Log buffer

Change-Id: I1aabb57f79bead726fcf36d59839702cd6a3521d
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>