stmmac: add the Energy Efficient Ethernet support

This patch adds the Energy Efficient Ethernet support to the stmmac.

Please see the driver's documentation for further details about this support
in the driver.

Thanks also goes to Rayagond Kokatanur for his first implementation.

Note:
 to clearly manage and expose the lpi interrupt status and eee ethtool
 stats I've had to do some modifications to the driver's design and I
 found really useful to move other parts of the code (e.g. mmc irq stat)
 in the main directly. So this means that some core has been reworked
 to introduce the EEE.

v1: initial patch
v2: fixed some sparse issues (typos)
v3: erroneously sent the v2 renamed as v3
v4:
	o Fixed the return value of the stmmac_eee_init as suggested by D.Miller
	o Totally reviewed the ethtool support for EEE
	o Added a new internal parameter to tune the SW timer for TX LPI.
v5: do not change any eee setting in case of the stmmac_ethtool_op_set_eee fails
    (it has to return -EOPNOTSUPP in that case).

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