Replace S-EL3 references by EL3

Replace S-EL3 references by EL3

The "Secure" prefix (S-ELx) is valid only for S-EL0 and S-EL1 but is
meaningless for EL3, since EL3 is always secure. Hence, the "S" prefix
has been removed from wherever it was used as "S-EL3".

Change-Id: Icdeac9506d763f9f83d7297c7113aec7b85e9dbe
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>