stmmac: adding DT parameter for LPI tx clock gating

This patch adds a new parameter to the stmmac DT: snps,en-tx-lpi-clockgating.
It was ported from synopsys/dwc_eth_qos.c and it is useful if lpi tx clock
gating is needed by stmmac users also.

Signed-off-by: Joao Pinto <jpinto@synopsys.com>
Tested-by: Niklas Cassel <niklas.cassel@axis.com>
Reviewed-by: Lars Persson <larper@axis.com>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>