lib: sbi: Make the root domain instance global variable

We make the the root domain instance global variable so that
platform support and drivers can iterate over the root domain
regions.

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