arm/arm64: smccc: Add SMCCC-specific return codes

We've so far used the PSCI return codes for SMCCC because they
were extremely similar. But with the new ARM DEN 0070A specification,
"NOT_REQUIRED" (-2) is clashing with PSCI's "PSCI_RET_INVALID_PARAMS".

Let's bite the bullet and add SMCCC specific return codes. Users
can be repainted as and when required.
Acked-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

include/linux/arm-smccc.h

@@ -291,5 +291,10 @@ asmlinkage void __arm_smccc_hvc(unsigned long a0, unsigned long a1,
  */
 #define arm_smccc_1_1_hvc(...)	__arm_smccc_1_1(SMCCC_HVC_INST, __VA_ARGS__)
 
+/* Return codes defined in ARM DEN 0070A */
+#define SMCCC_RET_SUCCESS			0
+#define SMCCC_RET_NOT_SUPPORTED			-1
+#define SMCCC_RET_NOT_REQUIRED			-2
+
 #endif /*__ASSEMBLY__*/
 #endif /*__LINUX_ARM_SMCCC_H*/