dt-bindings: can: xilinx_can: Add reset description

IP core has input for reset signal which can be connected that's why
describe optional reset property.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/all/bfaed896cc51af02fe5f290675313ab4dcab0d33.1689164442.git.michal.simek@amd.com


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