lib: sbi: Extend sbi_hartmask to support both hartid and hartindex

Currently, the sbi_hartmask is indexed by hartid which puts a
limit on hartid to be less than SBI_HARTMASK_MAX_BITS.

We extend the sbi_hartmask implementation to use hartindex and
support updating sbi_hartmask using hartid. This removes the
limit on hartid and existing code works largely unmodified.

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