lib: sbi: Simplify the initialization of root_hmask in sbi_domain_init

The original code has multiple conversions between hartid and
hartindex. Can call sbi_hartmask_set_hartindex directly to
avoid conversion.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>