Documentation: LoongArch: Add basic documentations

Add some basic documentation for LoongArch. LoongArch is a new RISC ISA,
which is a bit like MIPS or RISC-V. LoongArch includes a reduced 32-bit
version (LA32R), a standard 32-bit version (LA32S) and a 64-bit version
(LA64).

Tested-by: Bagas Sanjaya <[email protected]>
Reviewed-by: Jiaxun Yang <[email protected]>
Co-developed-by: WANG Xuerui <[email protected]>
Signed-off-by: WANG Xuerui <[email protected]>
Signed-off-by: Huacai Chen <[email protected]>

Author: chenhuacai

Documentation/arch.rst

@@ -13,6 +13,7 @@ implementation.
    arm/index
    arm64/index
    ia64/index
+   loongarch/index
    m68k/index
    mips/index
    nios2/index

Documentation/loongarch/features.rst

@@ -0,0 +1,3 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. kernel-feat:: $srctree/Documentation/features loongarch

Documentation/loongarch/index.rst

@@ -0,0 +1,21 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================
+LoongArch Architecture
+======================
+
+.. toctree::
+   :maxdepth: 2
+   :numbered:
+
+   introduction
+   irq-chip-model
+
+   features
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`