Add missing readme for pre-built non-PSA secure

Author: ccli8

targets/TARGET_NUVOTON/TARGET_M2351/TARGET_M23_NS/TARGET_NU_PFM_M2351_NS/TARGET_NU_PREBUILD_SECURE/README.md

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+The M2351 is a TrustZone target that requires two codes: secure and nonsecure. Secure code runs first to set up the secure environment and then brings up the nonsecure code. `NuMaker-mbed-TZ-secure-example.hex` is the pre-built secure image, and `cmse_lib.o` is its accompanying
+secure gateway library, which exports secure functions to nonsecure code. It is built from the [non-PSA secure code template](https://github.com/OpenNuvoton/NuMaker-mbed-TZ-secure-example). Because secure and nonsecure code may start at an address other than `0x0`, we have chosen `.hex` rather than `.bin` as the output format.
+
+## Hardware partition
+
+On TrustZone targets, you need to partition the hardware first for secure code to run on secure world and nonsecure code to run on nonsecure world.
+
+The pre-built secure code has the following hardware partition:
+
+- Flash (512KiB in total): 256KiB for secure and 256KiB for nonsecure.
+- SRAM (96KiB in total): 32KiB for secure and 64KiB for nonsecure.
+- Peripherals: Most are configured to nonsecure except the following, which are hardwired or reserved:
+    - **SYS/CLK** hardwired to secure. Accessible to nonsecure through a secure gateway.
+    - **FMC** hardwired to secure. Accessible to nonsecure through a secure gateway.
+    - **WDT** hardwired to secure. Accessible to nonsecure through a secure gateway.
+    - **RTC** configured to secure. Accessible to nonsecure through a secure gateway.
+    - **TMR0/1** hardwired to secure. TMR0 implements secure `us_ticker`, and TMR1 implements secure `lp_ticker`.
+    - **TMR2/3** configured to nonsecure. TMR2 implements nonsecure `us_ticker`, and TMR3 implements nonsecure `lp_ticker`.
+    - **PDMA0** hardwired to secure. Implements secure asynchronous transfer.
+    - **PDMA1** configured to nonsecure. Implements nonsecure asynchronous transfer.
+
+## Pre-built secure code files
+
+-   NuMaker-mbed-TZ-secure-example.hex
+    Pre-built secure image generated in non-PSA secure target build and to combine with non-secure image in non-PSA non-secure target post-build
+-   cmse_lib.o
+    Pre-built secure gateway library generated in non-PSA secure target build and to link with non-secure code in non-PSA non-secure target build
+
+## Reference
+
+Please refer to the [non-PSA secure code template](https://github.com/OpenNuvoton/NuMaker-mbed-TZ-secure-example) for details.