doc: Update mentions of README.fdt-control

Update documents 'README.fdt-control' reference to replacement 'control.rst':
  doc/arch/nios2.rst
  dts/Makefile

Also convert some adjacent pathname mentions to rST links where applicable

Fixes: 3e9fddfc4f14 "doc: Move devicetree control doc to rST"
Signed-off-by: E Shattow <e@freeshell.de>
6 files changed