[M2351] Add README.md for secure library/executable

Author: ccli8

targets/TARGET_NUVOTON/TARGET_M2351/TARGET_NUMAKER_PFM_M2351/TARGET_M23_NS/README.md

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+# Default secure code for M2351
+
+M2351 is a TrustZone target which requires two codes: secure and non-secure.
+Secure code would run first to set up secure environment and then bring up non-secure code.
+`NuMaker-mbed-TZ-secure-example.hex` is the default secure code and `cmse_lib.o` is its accompanying
+secure gateway library which exports secure functions to non-secure code. It is built from
+[secure code template](https://github.com/OpenNuvoton/NuMaker-mbed-TZ-secure-example).
+With secure/non-secure code possibly not starting at address `0x0`, `.hex` rather than `.bin` is
+chosen as output format.
+
+## Hardware partition
+
+On TrustZone targets, it is necessary to partition the hardware first for secure/non-secure code
+to run on secure/non-secure worlds respectively.
+
+The default secure code has the following hardware partition:
+- Flash (512KiB in total): 256KiB/256KiB to secure/non-secure respectively
+- SRAM (96KiB in total): 32KiB/64KiB to secure/non-secure respectively
+- Peripherals: most are configured to non-secure except the following which are hardwired or reserved:
+    - **SYS/CLK** hardwired to secure. Accessible to non-secure through secure gateway
+    - **FMC** hardwired to secure. Accessible to non-secure through secure gateway
+    - **WDT** hardwired to secure. Accessible to non-secure through secure gateway
+    - **RTC** configured to secure. Accessible to non-secure through secure gateway
+    - **TMR0/1** hardwired to secure. Implement secure `us_ticker`/`lp_ticker` respectively
+    - **TMR2/3** configured to non-secure. Implement non-secure `us_ticker`/`lp_ticker` respectively
+    - **PDMA0** hardwired to secure. Implement secure asynchronous transfer
+    - **PDMA1** configured to non-secure. Implement non-secure asynchronous transfer
+
+## Flash secure/non-secure code
+
+To flash secure/non-secure code on M2351, drag-n-drop `NuMaker-mbed-TZ-secure-example.hex`
+first and then built user program e.g. [mbed-os-example-blinky.hex](https://github.com/ARMmbed/mbed-os-example-blinky).
+
+## Reference
+
+Refer to [secure code template](https://github.com/OpenNuvoton/NuMaker-mbed-TZ-secure-example) for details.