net: ethernet: ti: cpts: add irq support

Add CPTS IRQ support, but do not enable it. By default, the CPTS driver
will continue working using polling mode which is required for CPTS to
continue working on platforms other than CPSW, like Keystone 2.

The CPTS IRQ support is required to enable support for HW_TS_PUSH events.
The CPSW CPTS IRQ and HW_TS_PUSH events support will be enabled in follow
up patches.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>