lib: sbi: Make sbi_domain_memregion_initfw() a local function

The sbi_domain_memregion_initfw() is no longer used outside
sbi_domain.c so let's make it a local function.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>