feat(fvp): add Event Log maximum size property in DT

feat(fvp): add Event Log maximum size property in DT

Updated the code to get and set the 'tpm_event_log_max_size' property
in the event_log.dtsi.

In this change, the maximum Event Log buffer size allocated by BL1 is
passed to BL2, rather than both relying on the maximum Event Log buffer
size macro.

Change-Id: I7aa6256390872171e362b6f166f3f7335aa6e425
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>