macb: Common code to enable ptp support for MACB/GEM

This patch does the following:
- MACB/GEM-PTP interface
- registers and bitfields for TSU
- capability flags to enable PTP per platform basis

Signed-off-by: Andrei Pistirica <andrei.pistirica@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>