can: rcar: use SPDX identifier for Renesas drivers

Adopt the SPDX license identifier headers to ease license compliance
management.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>