sbi: sbi_domain_context: Add spinlock for updating domain assigned_harts

Add spinlock protection to avoid race condition on assigned_harts
during domain context switching. Also, rename/add variables for
accessing the corresponding domain of target/current context.

Signed-off-by: Alvin Chang <alvinga@andestech.com>
Reviewed-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>