test: Added a simple synthetic benchmark for G2D

It measures MPix/s numbers for blit and fill operations done
by G2D, and also for comparison tests the performance of the
same operations done by pixman (software rendering).

G2D has clock frequency configured to be half of the RAM clock
frequency. So for 480 MHz RAM, we have G2D clocked at 240 MHz,
which means that no more than 240 MPix can be processed per
second. Unfortunately this limits the performance of a simple
operation such as solid fill.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>