target-mips/dsp_helper.c: Add ifdef guards around various functions

Add ifdef TARGET_MIPS64 guards around various functions that are only
called from helpers for TARGET_MIPS64 CPUs; this avoids compiler
warnings when building other configs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
