firmware: Fix the failure of scm call to update LVL_SHIFTER register

Currently only one argument is being passed in scm_call to update
LVL_SHIFTER register, but tz side expects two arguments to be
passed. This is leading to scm call failure. So fix this by adding
another argument and initializing it to zero which has no operation
to perform.

Change-Id: Ibe5b4a1635d58102b747926db65677251186ccba
Signed-off-by: Rohith Kollalsi <rkollals@codeaurora.org>