lib: Add sbi_init_count() API

We add sbi_init_count() API which provides number of times a
given HART completed init sequence (warmboot/coldboot).

This will be very useful in debugging. With upcoming SBI HSM
extension, it will also help in implementing one-time init
code for each HART.

Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Atish Patra <atish.patra@wdc.com>