Documentation: Move devicetree documentation

Move devicetree.md from acpi/ to getting_started/. The devicetree has
nothing to do with ACPI and getting_started has the most similar
information about coreboot.

Change-Id: I873b293f036a9e3bcdc98135386f9158c645513c
Signed-off-by: Marc Jones <marc@marcjonesconsulting.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66916
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

Documentation/acpi/index.md

@@ -10,7 +10,3 @@ upwards.
 ## GPIO
 
 - [GPIO toggling in ACPI AML](gpio.md)
-
-## devicetree
-
-- [Adding devices to a device tree](devicetree.md)

Documentation/getting_started/index.md

@@ -6,3 +6,4 @@
 * [Kconfig](kconfig.md)
 * [Writing Documentation](writing_documentation.md)
 * [Setting up GPIOs](gpio.md)
+* [Adding devices to a device tree](devicetree.md)