doc: reference: pm: move terms to glossary

Glossary is a place where all terms can be defined, so move them there.
Some terms have been slightly adjusted and SOC interface remove since it
was not much relevant.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

doc/glossary.rst

@@ -49,6 +49,16 @@ Glossary of Terms
       specified by the build system can be over-ridden by the application,
       if desired.
 
+   device runtime power management
+      Device Runtime Power Management (PM) refers the capability of devices to
+      save energy independently of the the system power state. Devices will keep
+      reference of their usage and will automatically be suspended or resumed.
+      This feature is enabled via the :kconfig:`CONFIG_PM_DEVICE_RUNTIME`
+      Kconfig option.
+
+   idle thread
+      A system thread that runs when there are no other threads ready to run.
+
    IDT
       (Interrupt Descriptor Table) a data structure used by the x86
       architecture to implement an interrupt vector table. The IDT is used
@@ -65,9 +75,17 @@ Glossary of Terms
       The set of Zephyr-supplied files that implement the Zephyr kernel,
       including its core services, device drivers, network stack, and so on.
 
+   power gating
+      Power gating reduces power consumption by shutting off areas of an
+      integrated circuit that are not in use.
+
    SoC
       `System on a chip`_
 
+   system power state
+      System power states describe the power consumption of the system as a
+      whole. System power states are are represented by :c:enum:`pm_state`.
+
    west
       A multi-repo meta-tool developed for the Zephyr project. See :ref:`west`.