riscv: sbi: Add IPI extension tests

Ensure IPIs directed at single harts are received and also that all
harts receive IPIs on broadcast. Also check for invalid param errors
when the params result in hartids greater than the max.

Signed-off-by: Cade Richard <cade.richard@berkeley.edu>
Co-developed-by: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>