lib: sbi: Override domain boot HART when coldboot HART assigned to it

If coldboot HART is assigned to a domain then we should use coldboot
HART as the boot HART of the domain. This allows coldboot HART to
continue boot sequence even when it is assigned to some non-root domain.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>