docs: add a memory-devices subdir to driver-api

There are two docs describing memory device drivers.

Add both to this new chapter of the driver-api.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/driver-api/index.rst

@@ -79,6 +79,7 @@ available subsections can be seen below.
    isapnp
    generic-counter
    lightnvm-pblk
+   memory-devices/index
    men-chameleon-bus
    ntb
    nvmem

Documentation/driver-api/memory-devices/index.rst

+=========================
+Memory Controller drivers
+=========================
+
+.. toctree::
+    :maxdepth: 1
+
+    ti-emif
+    ti-gpmc
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`

Documentation/memory-devices/ti-emif.rst → Documentation/driver-api/memory-devices/ti-emif.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 ===============================
 TI EMIF SDRAM Controller Driver

Documentation/bus-devices/ti-gpmc.rst → Documentation/driver-api/memory-devices/ti-gpmc.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 ========================================
 GPMC (General Purpose Memory Controller)