dt-bindings: aspeed-lpc: Add reset controller

This describes the reset controller present in the LPC address space.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
[p.zabel@pengutronix.de: removed a space before tab in indent]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>