lib: sbi_pmu: Use heap for per-HART PMU state

Instead of using a global array for per-HART PMU state, we should
use heap to on-demand allocate per-HART PMU state when the HART
is initialized in cold boot or warm boot path.

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