lib: Add timer exit API

We add sbi_timer_exit() API for OpenSBI exit path handling in
sbi_exit() implementation.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>

lib/sbi/sbi_init.c

@@ -205,6 +205,8 @@ void __noreturn sbi_exit(struct sbi_scratch *scratch)
 
 	sbi_platform_early_exit(plat);
 
+	sbi_timer_exit(scratch);
+
 	sbi_platform_final_exit(plat);
 
 	sbi_hart_hang();