This has nothing to do with Cortex-M and/or TrustZone. Your're better off posting in the right forum.
Anyway: It depends on your OS. If you run it baremetal you start in Aarch64 mode. If you run it in 32bit Android/Linux, then there is no way.
Since Snapdragon 450 uses Cortex-A53, the CPU hardware does support both 32 bit and 64 bit mode. But the SoC designer can tie a signal to make Cortex-A53 run in 32 bit mode in EL3, the consequence is the whole system would run in 32 bit mode (kernel, android, application). Or it could simply mean that the linux kernel is still 32 bit kernel,
View all questions in TrustZone for Armv8-M forum