Documentation: ACPI: move lpit.txt to firmware-guide/acpi and convert to reST

This converts the plain text documentation to reStructuredText format
and adds it to Sphinx TOC tree.

No essential content change.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/firmware-guide/acpi/index.rst

@@ -22,3 +22,4 @@ ACPI Support
    gpio-properties
    i2c-muxes
    acpi-lid
+   lpit

Documentation/firmware-guide/acpi/lpit.rst

@@ -0,0 +1,33 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===========================
+Low Power Idle Table (LPIT)
+===========================
+
+To enumerate platform Low Power Idle states, Intel platforms are using
+“Low Power Idle Table” (LPIT). More details about this table can be
+downloaded from:
+http://www.uefi.org/sites/default/files/resources/Intel_ACPI_Low_Power_S0_Idle.pdf
+
+Residencies for each low power state can be read via FFH
+(Function fixed hardware) or a memory mapped interface.
+
+On platforms supporting S0ix sleep states, there can be two types of
+residencies:
+
+  - CPU PKG C10 (Read via FFH interface)
+  - Platform Controller Hub (PCH) SLP_S0 (Read via memory mapped interface)
+
+The following attributes are added dynamically to the cpuidle
+sysfs attribute group::
+
+  /sys/devices/system/cpu/cpuidle/low_power_idle_cpu_residency_us
+  /sys/devices/system/cpu/cpuidle/low_power_idle_system_residency_us
+
+The "low_power_idle_cpu_residency_us" attribute shows time spent
+by the CPU package in PKG C10
+
+The "low_power_idle_system_residency_us" attribute shows SLP_S0
+residency, or system time spent with the SLP_S0# signal asserted.
+This is the lowest possible system power state, achieved only when CPU is in
+PKG C10 and all functional blocks in PCH are in a low power state.