serial: 8250_of: Add reset support

This adds the hooks for an optional reset controller in the 8250 device
tree node.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>