cmd: sbi: add Performance Monitoring Unit Extension

Version 1.0-rc3 of the RISC-V Supervisor Binary Interface Specification
has added the Performance Monitoring Unit Extension.

The sbi command should be able to detect it.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>