With this change driver will benefit from existing phylib and thus
custom phy functionality implemented in the driver will go away:
 * Instantiation of the driver is now much shorter - 2 parameters
instead of 4.
 * Simplified phy management/functoinality in driver is replaced with
rich functionality of phylib.
 * Support of custom phy initialization is now done with existing
"board_phy_config".

Note that after this change some previously used config options
(driver-specific PHY configuration) will be obsolete and they are simply
substituted with similar options of phylib.

For example:
 * CONFIG_DW_AUTONEG - no need in this one. Autonegotiation is enabled
by default.
 * CONFIG_DW_SEARCH_PHY - if one wants to specify attached phy
explicitly CONFIG_PHY_ADDR board config option has to be used, otherwise
automatically the first discovered on MDIO bus phy will be used

I believe there's no need now in "doc/README.designware_eth" because
user only needs to instantiate the driver with "designware_initialize"
whose prototype exists in "include/netdev.h".

Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Vipin Kumar <vipin.kumar@st.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Mischa Jonker <mjonker@synopsys.com>
Cc: Shiraz Hashim <shiraz.hashim@st.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Amit Virdi <amit.virdi@st.com>
Cc: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>