lib: sbi: Introduce HART index in sbi_scratch

We introduce HART index and related helper functions in sbi_scratch
where HART index is contiguous and each HART index maps to a physical
HART id such that 0 <= HART index and HART index < SBI_HARTMASK_MAX_BITS.

The HART index to HART id mapping follows the index2id mapping provided
by the platform. If the platform does not provide index2id mapping then
identity mapping is assumed.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>