linux-aspeed: ast2600: Disable new dev node

Userspace has not yet adapted to the new layout. This should be
re-enabled in the future when pdbg has gained support for the /dev/fsi/
layout in a release.

(From meta-aspeed rev: 6d2917deee778a66dc4c7fb06b869197430b7664)

Change-Id: Ib9a0fb6ca18c50ec7dae9c7d482b385ed5f6af44
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>