mfd: dt: Add bindings for the Aspeed LPC Host Controller (LHC)

The LPC bus pinmux configuration on fifth generation Aspeed SoCs depends
on bits in both the System Control Unit and the LPC Host Controller.

The Aspeed LPC Host Controller is described as a child node of the
LPC host-range syscon device for arbitration of access by the host
controller and pinmux drivers.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>