include: sbi: Add domains_init() platform operation

We introduce domains_init() platform operation which can be used by
platform support to initialize/populate domains in the coldboot path.

The domains_init() is called late in the coldboot sequence from the
sbi_domain_finalize() so sbi_printf() can be used by platform support
to print errors/warnings at time of populating domains.

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>