blackfin: ezbrd: Remove non-functional DSA/KSZ8893M code

There is no in tree driver for the KSZ8893M switch driver, so just get rid of
the code in that board file.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>