doc: Migrate Renesas board docs to rst

Some of the information in README.rmobile is obsolete, references
defconfigs which no longer exist in u-boot or has broken links. The
information which is still relevant is moved into the reStructuredText
documentation under `doc/board/renesas`, and `doc/README.rmobile` is
dropped.

The list of boards in `doc/board/renesas` is converted into a table so
it's easier to see which defconfig to use. The list is expanded based on
reviewing the current u-boot code and the contents of the eLinux wiki
[1] [2].

[1]: https://elinux.org/R-Car
[2]: https://elinux.org/RZ-G



Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>