docs: riscv: convert boot-image-header.txt to ReST

docs: riscv: convert boot-image-header.txt to ReST

Convert this small file to ReST format by:
   - Using a proper markup for the document title;
   - marking a code block as such;
   - use tags for Author and date;
   - use tables for bit map fields.

While here, fix a broken reference for a document with is
planned but is not here yet.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>