The Trusted Random Number Generator service is using the standard SMC
service dispatcher, running in BL31. For that reason we list the files
implementing the service in bl31.mk.
However when building for a 32-bit TF-A runtime, sp_min.mk is the
Makefile snippet used, so we have to add the files into there as well.

This fixes 32-bit builds of platforms that provide the TRNG service.

Change-Id: I8be61522300d36477a9ee0a9ce159a140390b254
Signed-off-by: Andre Przywara <andre.przywara@arm.com>