ARM: rmobile: Add missing config bits

The commit "fb82fe38 configs: Resync defconfigs" resynced all config
files.  This exposed two latent issues with the Gen3 boards in that we
had not been setting CONFIG_DEFAULT_FDT_FILE correctly and had not been
setting CONFIG_CMD_MMC.  Fix both of these.

Fixes: fb82fe38 ("configs: Resync defconfigs")
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Signed-off-by: Tom Rini <trini@konsulko.com>