net: phy: make autonegotiation configurable

net: phy: make autonegotiation configurable

Introduce environment variables to configure the use of
ethernet autonegotiation.
The variable names are derived from the ethernet device name
('FEC' for TX51,TX53,TX6;  'FEC0','FEC1' for TX28; 'cpsw' for TX48)
with the suffix '_aneg' appended.
If the '*_aneg' variable is unset or set to a string starting in
'y','t' or '1' autonegotiation will be enabled and the '*_speed' and
'*_duplex' variables have no meaning.
Otherwise autonegotiation is disabled and the link speed and duplex
settings can be configured with the variables: '*_speed' and
'*_duplex' (with '*' replaced with the prefix described above).