meta-nuvoton: npcm8xx: set optee-os 3.18.0 as the preferred version

The latest arm/optee support 3.19, but we don't have 3.19 support yet.

Currently, we need to use 3.18 as preferred version for build pass.

Once we have support 3.19 then we will update new bbappend for 3.19.

Signed-off-by: Tim Lee <timlee660101@gmail.com>
Change-Id: I02bc26d05ee4f647c2cd2803a9ce1488dcfc738d
diff --git a/meta-nuvoton/conf/machine/include/npcm8xx.inc b/meta-nuvoton/conf/machine/include/npcm8xx.inc
index e8d5587..d3cb516 100644
--- a/meta-nuvoton/conf/machine/include/npcm8xx.inc
+++ b/meta-nuvoton/conf/machine/include/npcm8xx.inc
@@ -38,6 +38,8 @@
 
 PREFERRED_VERSION_trusted-firmware-a = "2.6.0"
 
+PREFERRED_VERSION_optee-os = "3.18.0"
+
 UBOOT_MKIMAGE:append:npcm8xx = " -E -B 8"
 
 COMPATIBLE_MACHINE:npcm8xx = "npcm8xx"