Update npu config for corstone-310

The NPU config used for Corstone-310 is Ethos-U55-256.

Change-Id: I386a8541ebe38b1dde4a0d3cf050e56c287815a1
diff --git a/targets/corstone-310/CMakeLists.txt b/targets/corstone-310/CMakeLists.txt
index 5583e3c..d9da888 100644
--- a/targets/corstone-310/CMakeLists.txt
+++ b/targets/corstone-310/CMakeLists.txt
@@ -26,7 +26,7 @@
     set(CMAKE_TOOLCHAIN_FILE "${CMAKE_CURRENT_SOURCE_DIR}/../../cmake/toolchain/armclang.cmake")
 endif()
 
-set(ETHOSU_TARGET_NPU_CONFIG "ethos-u55-128" CACHE STRING "NPU configuration")
+set(ETHOSU_TARGET_NPU_CONFIG "ethos-u55-256" CACHE STRING "NPU configuration")
 set(ETHOSU_TARGET_NPU_COUNT 1 CACHE INTERNAL "Number of NPUs")
 set(ETHOSU_TARGET_NPU_TA_COUNT 2 CACHE INTERNAL "Number of timing adapters per NPU")