sys: Add an SMCCC Random Number Generator driver

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 a

sys: Add an SMCCC Random Number Generator driver

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.

Reviewed by: cem, markm
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46989

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