ARMv8/PSCI: Fixup the device tree for PSCI

Set the enable-method in the cpu node to PSCI, and create device
node for PSCI, when PSCI was enabled.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>
diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
index 0e05e87..f406419 100644
--- a/arch/arm/lib/Makefile
+++ b/arch/arm/lib/Makefile
@@ -55,6 +55,8 @@
 obj-y	+= cache-cp15.o
 endif
 
+obj-y	+= psci-dt.o
+
 obj-$(CONFIG_DEBUG_LL)	+= debug.o
 
 # For EABI conformant tool chains, provide eabi_compat()