imx: add cpu-freq SIP runtime service support

On i.MX8QM/i.MX8QX with system controller inside, the CPU's clock
rate is managed by SCFW(system controller firmware) and can ONLY be
changed from secure world, so SIP runtime service is needed for
setting CPU's clock rate, this patch adds cpu-freq SIP runtime service
support.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
diff --git a/plat/imx/common/imx_sip_svc.c b/plat/imx/common/imx_sip_svc.c
index 23ff0bb..89b9df8 100644
--- a/plat/imx/common/imx_sip_svc.c
+++ b/plat/imx/common/imx_sip_svc.c
@@ -29,6 +29,9 @@
 #if (defined(PLAT_IMX8QM) || defined(PLAT_IMX8QX))
 	case  IMX_SIP_SRTC:
 		return imx_srtc_handler(smc_fid, handle, x1, x2, x3, x4);
+	case  IMX_SIP_CPUFREQ:
+		SMC_RET1(handle, imx_cpufreq_handler(smc_fid, x1, x2, x3));
+		break;
 #endif
 	default:
 		WARN("Unimplemented i.MX SiP Service Call: 0x%x\n", smc_fid);