Documentation: gpio: Move legacy documentation to driver-api

Move gpio/gpio-legacy.txt to driver-api/gpio/legacy.rst and make sure it
builds cleanly as ReST.

Also move the legacy API reference from index.rst to legacy.rst.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/gpio/index.rst

@@ -9,6 +9,7 @@ Contents:
 
    intro
    driver
+   legacy
 
 Core
 ====
@@ -19,15 +20,6 @@ Core
 .. kernel-doc:: drivers/gpio/gpiolib.c
    :export:
 
-Legacy API
-==========
-
-The functions listed in this section are deprecated. The GPIO descriptor based
-API described above should be used in new code.
-
-.. kernel-doc:: drivers/gpio/gpiolib-legacy.c
-   :export:
-
 ACPI support
 ============