dm: core: Fix Kconfig text to mention SPL in SPL_OF_TRANSLATE

Add a remark about SPL to this Kconfig option. Otherwise its identitcal
to the non-SPL version, which is confusing.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>