at91: Missing kernel config symbols for AT91 target platform

I looked into buildbot snapshots and noticed that some platforms have
not successfully compiled for a long time. One example is at91, which
seems to be missing device-related kernel config symbols and the
snapshot build crashes already there.

Please find attached a patch that makes the at91 kernel config and
build to succeed again.

Signed off by: hannu.nyman@iki.fi

SVN-Revision: 31288