Aspeed UART routing driver

[Oskar Senft]

Hi

As discussed with Andrew, we have a use case that requires the UART
routing on the AST2500 to be configured at runtime.

After talking with Linus Walleij, we concluded that the pinmux driver is
not a good match for this use case.

I went ahead and implemented a "UART routing control driver" for the
AST2500. It may just work on the AST2400 as is, but I don't have a
system to test on.

Thanks
Oskar.