CAN bus tests

CAN bus tests

This commit for testing CAN modules, sending and receiving frames, showing stats and and testing suitable bitrates for transmission.
The "test_can_loopback_mode" test can be run on a board with one interface.
This test will fail gracfully if the board dosen't have any CAN interface.
The other tests needs to be run on a board with two CAN interfaces (e.g. dra7 with external transceiver.)
These tests requires two interfaces on the same board to send frames with the first interface and to receive them with the other one
These tests will fail gracfully by skiping all test cases if the board doesn't have two interfaces.
This link for the DRA7xx-EVM job : http://lava.baylibre.com:10080/scheduler/job/2860
This link for the AM335X-BONEBLACK (no CAN interface) : http://lava.baylibre.com:10080/scheduler/job/2862

Change-Id: I7d4c69f7817adbea8281d264cbbb7fa55571584e
Signed-off-by: khouloud touil <ktouil@baylibre.com>