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