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

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>