ARM: dts: aspeed: Move pinctrl subnodes to improve readability

Moving the subnodes out of the pinctrl node declaration to a reference
allows easier access to the remaining parts of the devicetree.
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Xo Wang <xow@google.com>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>