buildroot

sunxi-mali-mainline-driver: set parallel build according to buildroot

sunxi-mali-mainline-driver builds using parallel compilation by default,
using -j $(nproc), but this could be not wanted.

Pass -j $(PARALLEL_JOBS) to build using jobs configured in buildroot.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>