MUL: Add right shift on 32x32 multiply

The result of 32x32 elementwise multiply
exceeds the int32_t result type range.
This change adds a right scaling shift
argument to shift down the result.

Change-Id: I6ae17e6dc3fe342d052304533158ad2d0e7bb7be
Signed-off-by: Dominic Symes <dominic.symes@arm.com>