ARM: configs: aspeed: Disable IPV6 SIT device

No one is using this device on OpenBMC systems, and there is no code to
manage it in phosphor-networkd (the default OpenBMC userspace) as of
March 2021:

> [...] if you don't add IPv6 addresses to the sit interface
> it doesn't do anything. The defacto way to do that on an interface in
> OpenBMC is to have it managed by phosphor-networkd. On top of this, to
> support sit you would need a way to configure the local / remote IPv4
> addresses used to back it.

Signed-off-by: Joel Stanley <joel@jms.id.au>