atmel_serial: enable PPS support

Enables PPS support in atmel serial driver to make PPS API working.

Signed-off-by: Viktar Palstsiuk <viktar.palstsiuk@promwad.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>