sandbox: serial: Support a coloured console

The current sandbox serial driver is a pretty trivial example and does not
have the featues that might be needed for other board serial drivers. To
help provide a better example, add a text colour property to the device
tree for sandbox. This uses platform data, a device tree node, driver
private data and a remove() method.

Signed-off-by: Simon Glass <sjg@chromium.org>