can: c_can: add support to 64 message objects

D_CAN controller supports 16, 32, 64 or 128 message objects, comparing
to 32 on C_CAN. AM335x/AM437x Sitara processors and DRA7 SOC all
instantiate a D_CAN controller with 64 message objects, as described
in the "DCAN features" subsection of the CAN chapter of their
technical reference manuals.

The driver policy has been kept unchanged, and as in the previous
version, the first half of the message objects is used for reception
and the second for transmission.

The I/O load is increased only in the case of 64 message objects,
keeping it unchanged in the case of 32. Two 32-bit read accesses are
in fact required, which however remained at 16-bit for configurations
with 32 message objects.

Link: https://lore.kernel.org/r/20210302215435.18286-7-dariobin@libero.it
Signed-off-by: Dario Binacchi <dariobin@libero.it>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>