ARM: dts: aspeed: Clean up AST2500 pinctrl properties

ARM: dts: aspeed: Clean up AST2500 pinctrl properties

Many platforms were specifying the `aspeed,external-nodes` property
required by the AST2500 pinctrl. However, its been specified in the
pinctrl node directly in aspeed-g5.dtsi for quite a long time now.

Remove the unnecessary override from all relevant platform dts files.

Link: https://lore.kernel.org/r/20240802-dt-warnings-bmc-dts-cleanups-v1-5-1cb1378e5fcd@codeconstruct.com.au
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>