drivers: can: Add xmc4xxx CAN support

Adds CAN drivers for XMC4xxx SoCs.

XMC4xxx has multiple CAN nodes. The nodes share a common clock and
a message object pool.

The CAN nodes do not have a loopback mode. Instead there is an
internal bus which can be used to exchange messages between
nodes on the SoC. For this reason tests/samples which rely on the
loopback feature have been disabled.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>