Tegra186: fix MISRA Rule 8.3 violation

MISRA Rule 8.3, All declarations of an object or function
shall use the same names and type qualifiers.

This patch removes unused function(s).

Change-Id: I90865c003d46f1dc08bfb5f4fe8a327ea42a2bb7
Signed-off-by: Anthony Zhou <anzhou@nvidia.com>

plat/nvidia/tegra/soc/t186/plat_psci_handlers.c

@@ -25,8 +25,6 @@
 #include <tegra_private.h>
 
 extern void memcpy16(void *dest, const void *src, unsigned int length);
-
-extern void prepare_cpu_pwr_dwn(void);
 extern void tegra186_cpu_reset_handler(void);
 extern uint64_t __tegra186_cpu_reset_handler_end,
 		__tegra186_smmu_context;