Blackfin arch: Add BF537-STAMP platform support for ENC28J60 SPI Ethernet MAC

This device shouldn't be considered as an alternative to a Memory Mapped
or built-in Ethernet MAC. Throughput is slow (~460kByte/s) while generating
a very high system load (~60%).

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>