lib: sbi_pmu: add callback for counter width

This patch adds a callback to fetch the number of bits implemented for a
custom firmware counter. If the callback fails or is not implemented then
width defaults to 63.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>