stmmac: Define CSUM offload engine Types

This patch explicitly defines the CSUM offload engine type which need
(not mandatory) to be passed from the platform code.
STMMAC core supports two check sum offload engine types- Type-1 & Type-2.
Also, there are STMMAC cores that do not have the check sum offload
capabilities.

The behaviour of Type-1 & Type-2 cores related to provision of checksum
increases the packet length for Type-1 cores by 2, as the checksum is appended
at the end of data packet and the same is made accountable in the DMA status.
The STMMAC cores beyond Version-3.5 provide HW interface registers which allows
the user to read the HW capabilities, while to support the previous cores the
information related to HW capabilities has to be provided from the platform
code.

The Type-1 cores which do not have the HW register interface need this
information.

This patch also updates the driver's doc.

Signed-off-by: Deepak Sikri <deepak.sikri@st.com>
Hacked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>