ARM: reference DPM defn from v6/v7 arch spec

The term "DPM" is probably not well known ("Device Power Management"?),
so identify its source in the current ARM architecture specification.
It's relevant to ARMv6, ARMv7-A, and ARMv7-R ... but not "M" profiles.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

src/target/arm_dpm.c

@@ -35,6 +35,12 @@
  * Implements various ARM DPM operations using architectural debug registers.
  * These routines layer over core-specific communication methods to cope with
  * implementation differences between cores like ARM1136 and Cortex-A8.
+ *
+ * The "Debug Programmers' Model" (DPM) for ARMv6 and ARMv7 is defined by
+ * Part C (Debug Architecture) of the ARM Architecture Reference Manual,
+ * ARMv7-A and ARMv7-R edition (ARM DDI 0406B).  In OpenOCD, DPM operations
+ * are abstracted through internal programming interfaces to share code and
+ * to minimize needless differences in debug behavior between cores.
  */
 
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