atmel_serial: show tty name in /proc/interrupts

When possible, pass the tty name to request_irq() so that the user can easily
distinguish the different serial ports in /proc/interrupts.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Cc: Andrew Victor <linux@maxim.org.za>
Tested-by: Marc Pignat <marc.pignat@hevs.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>