dt-bindings: aspeed-lpc: Document LPC Host Interface Controller

The LPC Host Interface Controller is part of a BMC SoC that is used for
communication with the host.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>