fix(debug): decouple "get_el_str()" from backtrace

get_el_str() was implemented under ENABLE_BACKTRACE macro but being
used at generic places too, this causes multiple definition of this
function.
Remove duplicate definition of this function and move it out of
backtrace scope. Also, this patch fixes a small bug where in default
case S-EL1 is returned which ideally should be EL1, as there is no
notion of security state in EL string.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ib186ea03b776e2478eff556065449ebd478c3538