dev/psci: Use the correct SMCCC TRNG field

[andrew]

Summary

The TRNG service will return random data in x3. Use the correct field
when filling the buffer in trng_read.

This driver has not been in any release, so no security advisory is
needed.

Sponsored by: Arm Ltd

[bz]

sys/dev/psci/smccc_trng.c

116

Why are we limiting ourselves to 64 bits when the TRNG can return 192 on SMC64? Or do I misread https://developer.arm.com/documentation/den0098/latest/ ?
it leads to the next question below:

125

While true for 64bit of entropy as limited above for SMC64, I assume we do not use this on SCM32 (seems we don't).

Can we extend this to allow up-to 192 bits of entropy and use A3..A2..A1 for [0..63][64..127][128..191]? Does it make sense to do so?

[andrew]

sys/dev/psci/smccc_trng.c

116

random_sources_feed only ever asks for 64-bits so it simplifies the code to just read that