stmmac: update normal descriptor structure (v2)

This patch updates the normal descriptor structure
to work fine on new GMAC Synopsys chips.

Normal descriptors were designed on the old MAC10/100
databook 1.91 where some bits were reserved: for example
the tx checksum insertion and rx checksum offload.

The patch maintains the back-compatibility with old
MAC devices (tested on STx7109 MAC10/100) and adds new
fields that actually new GMAC devices can use.

For example, STx7109 (MAC10/100) will pass from the platform
  tx_coe = 0, enh_desc = 0, has_gmac = 0.
A platform like Loongson1B (GMAC) will pass:
  tx_coe = 1, enh_desc = 0, has_gmac = 1.

Thanks to Kelvin, he enhanced the normal descriptors for
GMAC (on MIPS Loongson1B platform).

Signed-off-by: Kelvin Cheung <keguang.zhang@gmail.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>