lib: utils: Fix sbi_hartid_to_scratch() usage in ACLINT drivers

The cold_init() functions of ACLINT drivers should skip the HART
if sbi_hartid_to_scratch() returns NULL because we might be dealing
with a HART that is disabled in the device tree.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>