Use the HSM SBI extension to start APs

The addition of the HSM SBI extension to OpenSBI introduces a new
breaking change: secondary harts will remain parked in the firmware,
until they are brought up explicitly via sbi_hsm_hart_start(). Add
the call to do this, sending the secondary harts to mpentry.

If the HSM extension is not present, secondary harts are assumed to be
released by the firmware, as is the case for OpenSBI =< v0.6 and BBL.

In the case that the HSM call fails we exclude the CPU, notify the
user, and allow the system to proceed with booting.

Reviewed by: markj (older version)
Differential Revision: https://reviews.freebsd.org/D24497