docs: early-userspace: move to driver-api guide

Those documents describe a kAPI. So, add to the driver-api
book.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/driver-api/index.rst

···
 
    basics
    infrastructure
+   early-userspace/index
    pm/index
    clk
    device-io

Documentation/early-userspace/index.rst Documentation/driver-api/early-userspace/index.rst

···
-:orphan:
-
 ===============
 Early Userspace
 ===============

Documentation/filesystems/nfs/nfsroot.txt

···
   A description of the process of mounting the root file system can be
   found in:
 
-    Documentation/early-userspace/early_userspace_support.rst
+    Documentation/driver-api/early-userspace/early_userspace_support.rst
 
 
 

Documentation/filesystems/ramfs-rootfs-initramfs.txt

···
   - The old initrd file was a gzipped filesystem image (in some file format,
     such as ext2, that needed a driver built into the kernel), while the new
     initramfs archive is a gzipped cpio archive (like tar only simpler,
-    see cpio(1) and Documentation/early-userspace/buffer-format.rst).  The
+    see cpio(1) and Documentation/driver-api/early-userspace/buffer-format.rst).  The
     kernel's cpio extraction code is not only extremely small, it's also
     __init text and data that can be discarded during the boot process.
 
···
 set permissions or create device nodes in the new archive.  (Note that those
 two example "file" entries expect to find files named "init.sh" and "busybox" in
 a directory called "initramfs", under the linux-2.6.* directory.  See
-Documentation/early-userspace/early_userspace_support.rst for more details.)
+Documentation/driver-api/early-userspace/early_userspace_support.rst for more details.)
 
 The kernel does not depend on external cpio tools.  If you specify a
 directory instead of a configuration file, the kernel's build infrastructure

usr/Kconfig

···
 	  When multiple directories and files are specified then the
 	  initramfs image will be the aggregate of all of them.
 
-	  See <file:Documentation/early-userspace/early_userspace_support.rst> for more details.
+	  See <file:Documentation/driver-api/early-userspace/early_userspace_support.rst> for more details.
 
 	  If you are not sure, leave it blank.