ARM: renesas: Deduplicate Renesas R-Car Gen3 defconfigs

ARM: renesas: Deduplicate Renesas R-Car Gen3 defconfigs

Deduplicate defconfigs for all Renesas R-Car Gen3 systems by factoring
out the common parts into generic renesas_rcar3.config and including
those using the #include <configs/...> preprocessor macro in the current
set of board specific defconfigs. The preprocessor macro is applicable
to defconfigs as well.

This introduces no functional change, the resulting .config is identical
for all Renesas R-Car Gen3 systems.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>