net: stmmac: Get correct timestamp values from XGMAC

TX Timestamp in XGMAC comes from MAC instead of descriptors. Implement
this in a new callback.

Also, RX Timestamp in XGMAC must be cheked against corruption and we need
a barrier to make sure that descriptor fields are read correctly.

Changes from v2:
	- Rework return code check (Jakub)
Changes from v1:
	- Rework the get timestamp function (David)

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>