serial: atmel: allow compile testing on non-atmel architectures

This commit also fixes compiler warnings and errors seen when building
on x86_64.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>