lib: sbi: Add function to register new domain

We add sbi_domain_register() function to help platform code register
a new domain explicitly instead of depending on domain_get() platform
callback. This function will eventually help us completely remove the
domain_get() platform callback.

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>