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/20190911165614.31641-2-joel@jms.id.au


Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>