da850evm: basic MII EMAC support

The current da850evm support in u-boot/master omits any use of
the davinci EMAC. This patch adds basic support for the EMAC using
the MII PHY found on the baseboard of the EVM. The MAC address is
read from the environment variable 'ethadd'. Note that this is
different from the da850evm support in the u-boot omapl1 tree
where the MAC address is read from SPI flash.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
CC: Sandeep Paulraj <s-paulraj@ti.com>
CC: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
CC: Ben Warren <biggerbadderben@gmail.com>
CC: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>