sys: Add an SMCCC Random Number Generator driver

[andrew]

Summary

The Arm True Random Number Generator Firmware Interface provides a way
to query the SMCCC firmware for up to 192 bits of entropy. Use it to
provide another source of randomness to the kernel.

Sponsored by: Arm Ltd

[andrew]

Remove an old TODO comment

[markm]

Looks OK to me.

[cem]

sys/dev/psci/smccc_trng.c

113

If this source supports 192 bits per request, why truncate to 64?

118

Earlier we use arm_smccc_invoke. What’s the difference?

[andrew]

• Check for SMCCC 1.1
• Remove an unused variable
• Use arm_smccc_invoke

[andrew]

sys/dev/psci/smccc_trng.c

113

It's simpler to do so and we only ever ask for 64 bits in random_sources_feed

[cem]

Thanks.