docs: add interrupts-target field to sp manifest

In order to support the ability to target a SPI interrupt to a
particular core, an impdef field "interrupts-target" has been added to
associate and interrupt id with an mpidr. The field is optional and if
not provided, existing SPMC behavior of routing to boot strap core is
maintained.

Signed-off-by: Raghu Krishnamurthy <raghu.ncstate@gmail.com>
Change-Id: I78ccfb45cd9b411cca4b36ff940064fc9dcd1622

docs/components/ffa-manifest-binding.rst

@@ -286,6 +286,18 @@ Device Regions
           - PPI:  0b01
           - SGI:  0b00
 
+- interrupts-target
+   - value type: <prop-encoded-array>
+   - A list of (id, mpdir upper bits, mpidr lower bits) tuples describing which
+     mpidr the interrupt is routed to, where:
+
+      - id: The <u32> interrupt ID. Must be one of those specified in the
+            "interrupts" field.
+      - mpidr upper bits: The <u32> describing the upper bits of the 64 bits
+                          mpidr
+      - mpidr lower bits: The <u32> describing the lower bits of the 64 bits
+                          mpidr
+
 - exclusive-access
    - value type: <empty>
    - Presence of this field implies that this endpoint must be granted exclusive