dt-bindings: riscv: remove obsolete cpus.txt

Remove the now-obsolete riscv/cpus.txt DT binding document, since we
are using YAML binding documentation instead.

While doing so, transfer the explanatory text about 'harts' (with some
edits) into the YAML file, at Rob's request.

Link: https://lore.kernel.org/linux-riscv/CAL_JsqJs6MtvmuyAknsUxQymbmoV=G+=JfS1PQj9kNHV7fjC9g@mail.gmail.com/


Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Rob Herring <robh@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>