ARM: renesas: Deduplicate common Renesas defconfig

ARM: renesas: Deduplicate common Renesas defconfig

Deduplicate common Renesas defconfig by factoring out the common
parts into generic renesas_rcar_defconfig 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 are
identical for all Renesas systems.

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