net: dsa: microchip: ptp: enable interrupt for timestamping

PTP Interrupt mask and status register differ from the global and port
interrupt mechanism by two methods. One is that for global/port
interrupt enabling we have to clear the bit but for ptp interrupt we
have to set the bit. And other is bit12:0 is reserved in ptp interrupt
registers. This forced to not use the generic implementation of
global/port interrupt method routine. This patch implement the ptp
interrupt mechanism to read the timestamp register for sync, pdelay_req
and pdelay_resp.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>