ARM: renesas: Rename board/renesas/rcar-common to board/renesas/common

ARM: renesas: Rename board/renesas/rcar-common to board/renesas/common

Rename directory board/renesas/rcar-common to board/renesas/common and
move files. This allows the build system to use HAVE_VENDOR_COMMON_LIB
which automatically includes board/$(VENDOR)/common/Makefile . Create
temporarily empty board/renesas/common/Makefile to be extended with
actual content later in this series. This is a preparatory patch for
board Makefile simplification. No functional change so far.

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>