ARM: dts: aspeed: Add G5 USB host pinmux

Set the default pinmux for EHCIs so boards don't have to do
it an document why it is not set for UHCI.

Remove the properties from the AST2500 EVB board which are
now redundant

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>