Elaborate on HAL - SPM Porting Guide

Author: danny4478

docs/porting/psa/spm.md

@@ -88,6 +88,11 @@ Target specific code of silicon partners who wish to have SPM capabilities must:
 - Implement a list of functions which are being called by SPM code
 - Call other functions supplied by ARM
 
+The HAL can be logically divided into 3 different fields:
+- **Addresses:** This part of HAL allows the silicon partner to share the addresses set in the linker scripts with the SPM code. The SPM uses these addresses mostly to enforce access permissions.
+- **Mailbox:** The mailbox is the mechanism for Inter Processor Communication. This part of HAL allows the silicon partner to implement specific parts of the mailbox mechanism according to the specification of the their specific platform.
+- **Secure Partition Environment:** This part of HAL allows the silicon partner to apply their specific memory protection scheme.
+
 A list of these functions can be found here [TODO: WHEN READY, ADD LINK TO DOXYGEN FILES OF HAL FUNCTIONS]