[TF-M] Slight update on design proposal process document

Hi,

I have made a slight update on the design proposal process document, to simplify the design document drafting.

The 'Author' and 'Organization' are not both mandatory, one of them can be optional depends on the contributor. And mark the 'status' optional since it can be covered by the document system.

Feel free to comment on the patch: https://review.trustedfirmware.org/c/trusted-firmware-m/+/3407

And I prefer to comment on the issue link since there is more better for discussion: https://developer.trustedfirmware.org/T673

Reply here is an option, too.

BR

/Ken