ARM/AVR32: get rid of serial_at91.h

The definitions provided by serial_at91.h are only used by the
atmel_serial driver, and the function that uses it is never called
from anywhere in the kernel.  Therefore, these definitions are unused
and/or obsolete, and can be removed.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>