lib: sbi_scratch: Introduce HART id to scratch table

Instead of calling hartid_to_scratch() callback every time when
we want sbi_scratch pointer from HART id, we create a table of
sbi_scratch pointers and use that to get sbi_scratch pointer.

As a result of HART id to scratch table, the conversion of
HART id to sbi_scratch pointer is just 2-3 instructions which
was 9 instructions previously.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>