net: ethernet: renesas: group entries in Makefile

A new Renesas driver shall be added soon. Prepare the Makefile by
grouping the specific objects to the Kconfig symbol for better
readability. Improve the file description a tad while here.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20231022205316.3209-2-wsa+renesas@sang-engineering.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>