fix(bl31): avoid clearing of argument registers in RESET_TO_BL31 case

It was assumed that BL31 is the first bootloader to run so there's
no argument to relay from a previous bootloader in RESET_TO_BL31
case, however this is not true for every platform with a non-TF-A
bootloader that might get executed before BL31 while compiling in
RESET_TO_BL31 feature.

Thus, by avoiding zeroing registers, the arguments passed from the
previous bootloader to BL31 are preserved.

Change-Id: I7bb66a10d1fd551ba3fd59a7a38ab5bde3197f72
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>

bl31/aarch64/bl31_entrypoint.S

@@ -66,17 +66,6 @@ func bl31_entrypoint
 		_init_c_runtime=1				\
 		_exception_vectors=runtime_exceptions		\
 		_pie_fixup_size=BL31_LIMIT - BL31_BASE
-
-	/* ---------------------------------------------------------------------
-	 * For RESET_TO_BL31 systems, BL31 is the first bootloader to run so
-	 * there's no argument to relay from a previous bootloader. Zero the
-	 * arguments passed to the platform layer to reflect that.
-	 * ---------------------------------------------------------------------
-	 */
-	mov	x20, 0
-	mov	x21, 0
-	mov	x22, 0
-	mov	x23, 0
 #endif /* RESET_TO_BL31 */
 
 	/* --------------------------------------------------------------------