Add config option for disabling DM9000-SROM support.

Some boards do not have SROM support for the DM9000 network adapter.
Instead of listing these board names in the driver code, make this
option configurable from the board config file.

It also removes a build warning for the at91sam9261ek board:
'dm9000x.c:545: warning: 'read_srom_word' defined but not used'

And it repaires the trizepsiv board build which was broken around the
same routines

Signed-off-by: Remy Bohmer <linux@bohmer.net>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>