at91: Add CAN init function

To enable CAN init, CONFIG_CAN has to be defined in the board config file
and at91_can_hw_init() has to be called in the board specific code.

CAN is available on AT91SAM9263 and AT91CAP9 SoC.

Signed-off-by: Daniel Gorsulowski <Daniel.Gorsulowski@esd.eu>