Looks like some similar discussion here:
https://patchwork.kernel.org/patch/3952471/
I made the same change and it's working fine now:
diff --git a/arch/arm/mach-omap2/omap_hwmod_33xx_data.c b/arch/arm/mach-omap2/om
index b298d6d..8044a3b 100644
--- a/arch/arm/mach-omap2/omap_hwmod_33xx_data.c
+++ b/arch/arm/mach-omap2/omap_hwmod_33xx_data.c
@@ -455,6 +455,7 @@ static struct omap_hwmod am33xx_debugss_hwmod = {
.name = "debugss",
.class = &am33xx_debugss_hwmod_class,
.clkdm_name = "l3_aon_clkdm",
+ .flags = HWMOD_INIT_NO_IDLE,
.main_clk = "trace_clk_div_ck",
.prcm = {
.omap4 = {