Enable Mbed OS PSA

README.md:
* Document the fact that this example enables Mbed OS PSA which
cannot coexist with TF-M.

atecc608a/mbed_app/json:
* Remove SAML21J18A which is not supported by the latest Mbed OS.
* Enable Mbed OS PSA required by the example.

Author: LDong-Arm

README.md

@@ -9,6 +9,9 @@ The examples in this repository demonstrate how to use the ATECC608A secure elem
 
 A target with I2C and power supply connections, connected to an ATECC608A secure element as shown in [Hardware interface](#hardware-interface).
 
+Note: This example enables Mbed OS PSA. It is _not_ suitable for or compatible with TF-M which has its own PSA implementation
+(e.g. Arm TrustZone on Arm v8-M).
+
 ## Mbed OS build tools
 
 ### Mbed CLI 2

atecc608a/mbed_app.json

@@ -1,18 +1,15 @@
 {
     "target_overrides": {
         "*": {
-            "target.features_add" : ["EXPERIMENTAL_API"],
+            "target.features_add" : ["EXPERIMENTAL_API", "PSA"],
+            "extra_labels_add": ["MBED_PSA_SRV"],
             "platform.stdio-baud-rate": 9600,
             "platform.stdio-convert-newlines": true,
             "mbed-trace.enable": 0
         },
         "NRF52_DK": {
             "cryptoauthlib.i2c_sda": "P0_26",
             "cryptoauthlib.i2c_scl": "P0_27"
-        },
-        "SAML21J18A": {
-            "cryptoauthlib.i2c_sda": "PA08",
-            "cryptoauthlib.i2c_scl": "PA09"
         }
     },
     "macros": [