aspeed: AST2500 Pinctrl Driver

This driver uses Generic Pinctrl framework and is compatible with
the Linux driver for ast2500: it uses the same device tree
configuration.

Not all pins are supported by the driver at the moment, so it actually
compatible with ast2400. In general, however, there are differences that
in the future would be easier to maintain separately.

Signed-off-by: Maxim Sloyko <maxims@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>