[NET] 5268/1: cs89x0: add support for i.MX31ADS ARM board.

Add support for i.MX31ADS board to the cs89x0 ethernet driver.

Rework Kconfig options for the cs89x0 driver to reduce the #ifdef
clutter.

Signed-off-by: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>