lib: sbi: Print platform PMU device at boot-time

Let us print the platform PMU device name at the boot-time so that users
know whether the underlying platform has custom per-HART PMU operations.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>