Add guidelines for porting CC310 on Mbed OS

Add guidelines for porting CC310 on a new platform

Author: Ron Eldor

features/mbedtls/targets/TARGET_CRYPTOCELL310/Readme.md

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+# Guidelines for porting Cryptocell to Mbed OS
+
+Cryptocell (CC) 310 is a Hardware accelerator that can be ported to several boards, assuming these boards have CC 310 embedded in their Hardware.  
+CC 310 driver is consisted of three libraries:
+
+* A common core library(`libcc_core.a`)
+* A platform specific trng library, containing trng related information, for sampling sufficient entropy on the specific platform(`libcc_trng.a`)
+* A platform specific library containig extra information, such as the CC register's base addresses on the specific board.(`libcc_ext.a`)
+
+In order to port your CC 310 driver to Mbed OS, on your specific target, do the following:
+
+1. In `targets.json` add the following  to your target:
+ 	* `MBEDTLS_CONFIG_HW_SUPPORT` to `macros_add` key.
+ 	* `CRYPTOCELL` to `device_has_add` key.
+ 	* `CRYPTOCELL310` to `extra_labels_add` key.
+1. In `objects.h` include `objects_cryptocell.h`.(You can condition it with `#if DEVICE_CRYPTOCELL` in case you have another `trng` engine for a differnt board, and `objects.h` is common file for your boards, in this case your common `trng_api.c` file should be compiled only if `#if !defined(DEVICE_CRYPTOCELL)`).
+1. In `features/mbedtls/targets/TARGET_CRYPTOCELL310/TARGET_<target name>` add your platform specific libraries for all toolchains in `TOOLCHAIN_ARM`, `TOOLCHAIN_GCC_ARM` and `TOOLCHAIN_IAR` respectively.
+1. Add your CC setup code:
+	* Implement `cc_platform_setup()` and `cc_platform_terminate()` to enable CC on your platform, in case you have such limitations. You can implement these functions as empty functions.
+	* Define `cc_platform_ctx` in `cc_platform.h` in a way that will suit your implementation.
+