Allow using TPM as entropy source.

TPM has a built-in RNG, with its own entropy source.
The driver was extended to harvest 16 random bytes from TPM every 10 seconds.
A new build option "TPM_HARVEST" was introduced - for now, however, it
is not enabled by default in the GENERIC config.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Reviewed by: markm, delphij
Approved by: secteam
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D19620