MAINTAINERS: split Renesas Ethernet drivers entry

Since the Renesas Ethernet Switch driver was added by Yoshihiro Shimoda,
I started receiving the patches to review for it -- which I was unable to
do, as I don't know this hardware and don't even have the manuals for it.
Fortunately, Shimoda-san has volunteered to be a reviewer for this new
driver, thus let's now split the single entry into 3 per-driver entries,
each with its own reviewer...

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/de0ccc1d-6fc0-583f-4f80-f70e6461d62d@omp.ru


Signed-off-by: Paolo Abeni <pabeni@redhat.com>