can: at91_can: Fix Kconfig dependencies

The at91_can driver is AT91-specific so it should depend on ARCH_AT91
rather than just ARM. Add COMPILE_TEST as an alternative, so that the
driver can still be build-tested elsewhere.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>