fix(el3_runtime): do not save scr_el3 during EL3 entry

fix(el3_runtime): do not save scr_el3 during EL3 entry

scr_el3 registers cannot be modified in lower ELs which means it retains
the same value which is stored in the EL3 cpu context structure for the
given world. So, we should not save the register when entering to EL3
from lower EL as we have the copy of it present in cpu context.

During EL3 execution SCR_EL3 value can be modifed for following cases
 1. Changes which is required for EL3 execution, this change is temp
    and do not need to be saved.
 2. Changes which affects lower EL execution, these changes need to be
    written to cpu context as well and will be retrieved when scr_el3
    is restored as part of exiting EL3

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I9cc984ddf50e27d09e361bd83b1b3c9f068cf2fd