lib: sbi: Add Zkr in hart extensions

- Add Zkr as extension in sbi_hart_extensions enum
- Return "zkr" string for Zkr extension from sbi_hart_extension_id2string

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

include/sbi/sbi_hart.h

@@ -40,6 +40,8 @@ enum sbi_hart_extensions {
 	SBI_HART_EXT_ZICNTR,
 	/** HART has Zihpm extension */
 	SBI_HART_EXT_ZIHPM,
+	/** HART has Zkr extension */
+	SBI_HART_EXT_ZKR,
 	/** Hart has Smcntrpmf extension */
 	SBI_HART_EXT_SMCNTRPMF,