ARM: dts: aspeed: Add AST2600 pinmux nodes

Add them in their own dtsi and include it in aspeed-g6.dtsi to isolate
the cruft.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Andrew Jeffery <[email protected]>
Signed-off-by: Joel Stanley <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>

Author: amboar