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

@@ -1,3 +1,9 @@
+.. 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:
@@ -8,11 +14,13 @@ Residencies for each low power state can be read via FFH
 
 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)
+
+  - 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:
+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