ARM: rcar_gen3: boot with new and legacy FDTs

Starting from kernel 5.5 FDT names were split and renamed to align with config options. This leading to boot failure of old or new Apertis images depending of naming schema used in fdtfile environment variable.

Added "legacy_mmc" boot entries allowing to try the legacy FDT naming schema in case of boot failure with default naming schema.

Added LEGACY_FDT_FILE Kconfig option -- it is needed to provide the name of legacy FDT file.

To enable the second try the variable named CONFIG_LEGACY_FDT_FILE should be defined. In case if it is not available or empty -- there will be no additional boot attempt.

Set default and legacy FDT names for boards r8a779{5,6}. Proper names were tested only with H3 and M3 boards.