feat(imx8ulp): enable the DDR frequency scaling support

Enable the DDR frequency scaling support on i.MX8ULP.
Normally, the freq_index define is as below:

 0: boot frequency;
 1: low frequency(PLL bypassed);
 2. high frequency(PLL ON).

Currently, DDR DFS only do frequency switching between
Low freq and high freq.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
Change-Id: I3acd8bdf75e2dd6dff645b9f597dcfc0a756c428
diff --git a/plat/imx/common/imx_sip_svc.c b/plat/imx/common/imx_sip_svc.c
index b6b8ff8..c625704 100644
--- a/plat/imx/common/imx_sip_svc.c
+++ b/plat/imx/common/imx_sip_svc.c
@@ -40,6 +40,8 @@
 	case IMX_SIP_HIFI_XRDC:
 		SMC_RET1(handle, imx_hifi_xrdc(smc_fid));
 		break;
+	case IMX_SIP_DDR_DVFS:
+		return dram_dvfs_handler(smc_fid, handle, x1, x2, x3);
 #endif
 #if defined(PLAT_imx8mq)
 	case IMX_SIP_GET_SOC_INFO: