docs: driver-api: Move bt8xxgpio to the gpio directory

Let's declutter Documentation/driver-api a bit.
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20191004164059.10397-1-j.neuschaefer@gmx.netSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/gpio/index.rst

@@ -13,6 +13,7 @@ Contents:
    board
    drivers-on-gpio
    legacy
+   bt8xxgpio
 
 Core
 ====

Documentation/driver-api/index.rst

@@ -69,7 +69,6 @@ available subsections can be seen below.
    fpga/index
    acpi/index
    backlight/lp855x-driver.rst
-   bt8xxgpio
    connector
    console
    dcdbas

drivers/gpio/Kconfig

@@ -1329,7 +1329,7 @@ config GPIO_BT8XX
 	  The card needs to be physically altered for using it as a
 	  GPIO card. For more information on how to build a GPIO card
 	  from a BT8xx TV card, see the documentation file at
-	  Documentation/driver-api/bt8xxgpio.rst
+	  Documentation/driver-api/gpio/bt8xxgpio.rst
 
 	  If unsure, say N.