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

bl31/aarch64/runtime_exceptions.S

@@ -493,15 +493,16 @@ smc_handler64:
 	msr	spsel, #MODE_SP_EL0
 
 	/*
-	 * Save the SPSR_EL3, ELR_EL3, & SCR_EL3 in case there is a world
+	 * Save the SPSR_EL3 and ELR_EL3 in case there is a world
 	 * switch during SMC handling.
 	 * TODO: Revisit if all system registers can be saved later.
 	 */
 	mrs	x16, spsr_el3
 	mrs	x17, elr_el3
-	mrs	x18, scr_el3
 	stp	x16, x17, [x6, #CTX_EL3STATE_OFFSET + CTX_SPSR_EL3]
-	str	x18, [x6, #CTX_EL3STATE_OFFSET + CTX_SCR_EL3]
+
+	/* Load SCR_EL3 */
+	mrs	x18, scr_el3
 
 	/* Clear flag register */
 	mov	x7, xzr