serial: atmel_serial: add poll_get_char and poll_put_char uart_ops

Permits using KGDB over the console with the atmel_serial driver.

[akpm@linux-foundation.org: s/barrier/cpu_relax/]
Signed-off-by: Albin Tonnerre <albin.tonnerre@free-electrons.com>
Acked-by: Andrew Victor <linux@maxim.org.za>
Acked-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>