can: at91_can: add id_table and convert prime mailbox constats to functions

This is the second of two patches converting the at91_can driver from a
compile time mailbox setup to a dynamic one.

This patch first adds a id_table to the platform driver. Depending on the
driver_data the constants for the mailbox setup is selected. Then all
remaining prime mailbox constants are converted to functions, using the
run time selected mailbox constants.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>