meta-nuvoton: set npcm8xx optee preferred version

Set optee-os-tadevkit, and optee-test version to 3.18.0 for Nuvoton
npcm8xx device.

Change-Id: I98a5637f2ad266d56d37df5cfa36b136bad76966
Signed-off-by: Brian Ma <chma0@nuvoton.com>
diff --git a/meta-nuvoton/conf/machine/include/npcm8xx.inc b/meta-nuvoton/conf/machine/include/npcm8xx.inc
index 16bce5b..3be5485 100644
--- a/meta-nuvoton/conf/machine/include/npcm8xx.inc
+++ b/meta-nuvoton/conf/machine/include/npcm8xx.inc
@@ -40,6 +40,8 @@
 PREFERRED_VERSION_trusted-firmware-a = "2.6.0"
 
 PREFERRED_VERSION_optee-os = "3.18.0"
+PREFERRED_VERSION_optee-os-tadevkit = "3.18.0"
+PREFERRED_VERSION_optee-test = "3.18.0"
 
 UBOOT_MKIMAGE:append:npcm8xx = " -E -B 8"