meta-nuvoton: trusted-firmware-a: fix the version at v2.9.0

Currently, openbmc upstream already update tfa to version v2.10.0
For compatibling with latest openbmc, we need to fix the version at v2.9.0. Otherwise, we will meet this kind of build error from upstream.

ERROR: No recipes in default available for:
openbmc/meta-nuvoton/dynamic-layers/arm-layer/recipes-bsp/
trusted-firmware-a/trusted-firmware-a_2.9.%.bbappend

Signed-off-by: Tim Lee <timlee660101@gmail.com>
Change-Id: If652ee5478cc9c42e833dd4d2a203d7d5ebddca1
diff --git a/meta-nuvoton/conf/machine/include/npcm8xx.inc b/meta-nuvoton/conf/machine/include/npcm8xx.inc
index 1605363..24956a2 100644
--- a/meta-nuvoton/conf/machine/include/npcm8xx.inc
+++ b/meta-nuvoton/conf/machine/include/npcm8xx.inc
@@ -43,6 +43,7 @@
 
 COMPATIBLE_MACHINE:npcm8xx = "npcm8xx"
 TFA_PLATFORM = "npcm845x"
+PREFERRED_VERSION_trusted-firmware-a ?= "2.9.%"
 
 # Nuvoton prefers optee for BL32.
 TFA_SPD = "opteed"