tpm: Register TPM as entropy source

TPM is defined as an entropy and is called every 10 seconds. However it
was not registered and calls were discarded.

Signed-off-by: Jean-François Hren <jean-francois.hren@stormshield.eu>

MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/1398