modules: hal_ethos_u: fix failure to override ETHOSU_TARGET_NPU_CONFIG

This fixes failure to override ETHOSU_TARGET_NPU_CONFIG in hal_ethos_u
due to CMP0126 not being NEW. See:
https://cmake.org/cmake/help/latest/policy/CMP0126.html#policy:CMP0126



Signed-off-by: Chun-Chieh Li <ccli8@nuvoton.com>