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>