serial: atmel: add support for new UART version

Starting with sama5d2, the new UART revision has an hardware timer. So, add it
to the IP detection code and set the "has_hw_timer" property for it.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Reported-by: David Mosberger <davidm@egauge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>