riscv: sbi: Prepare for assembly entry points

The HSM tests will need to test HSM start and resumption from HSM
suspend. Prepare for these tests, as well other tests, such as the
SUSP resume tests, by providing an assembly file for SBI tests.

Signed-off-by: James Raphael Tiovalen <jamestiotio@gmail.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>