of: Add bindings for chosen node, stdout-path

Add a global binding for the chosen node.  Include a description of the
stdout-path, and an explicit statement on its extra options in the
context of a UART console.

Opening description stolen from www.devicetree.org, and part of the
remaining text provided by Mark Rutland.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
[grant.likely: remove reference to uart_parse_options]
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>