rockchip: rk3568-nanopi-r5: Disable SPL_DM_WARN Kconfig option

With the commit 6afdb158 ("dm: core: migrate debug() messages to use
dm_warn") use of DM_WARN/SPL_DM_WARN print a lot of debug messages.

Disable the SPL_DM_WARN Kconfig option to remove verbose logging and
restore normal serial console output during boot.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>