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>

include/sbi/sbi_domain.h

@@ -80,6 +80,9 @@ struct sbi_domain {
 	bool system_reset_allowed;
 };
 
+/** The root domain instance */
+extern struct sbi_domain root;
+
 /** HART id to domain table */
 extern struct sbi_domain *hartid_to_domain_table[];