[U-Boot] ns16550: change reg-shift property default to zero

[Stephen Warren <swarren@wwwdotorg.org>]

On 12/06/2015 03:07 PM, Tom Rini wrote:
> On Sun, Nov 29, 2015 at 02:01:03PM +0800, Thomas Chou wrote:
>
>> Change reg-shift property default to zero. When the integer property
>> is missing, it should be taken as zero. This is consistent to Linux
>> drivers/tty/serial/of_serial.c.

It's not generally true that missing properties have value zero. Rather, 
whatever values was assumed by the semantics of the binding before that 
optional property was defined should be assumed.

What the Linux kernel driver does also isn't justification for this 
change, since DT bindings define how they work, rather than a particular 
OS implementation forcing the hand of the binding.

The DT binding documentation must state the default value/semantics for 
any optional property. Can you please make sure the DT binding 
documentation is updated to describe this case?

(Note that I have no objection to this patch; the actual change seems 
fine. It's simply that the justifications given in the patch description 
for it aren't entirely robust.)