dm: doc: Fix serial howto u-boot,dm-pre-reloc typo

In a couple of places the document says u-boot,pre-reloc but all
examples show u-boot,dm-pre-reloc, use the latter consistently.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/driver-model/serial-howto.rst b/doc/develop/driver-model/serial-howto.rst
index 9da0e57..5b1d57d 100644
--- a/doc/develop/driver-model/serial-howto.rst
+++ b/doc/develop/driver-model/serial-howto.rst
@@ -62,7 +62,7 @@
 Here are some things you might need to consider:
 
 1. The serial driver itself needs to be present before relocation, so that the
-   U-Boot banner appears. Make sure it has a u-boot,pre-reloc tag in the device
+   U-Boot banner appears. Make sure it has a u-boot,dm-pre-reloc tag in the device
    tree, so that the serial driver is bound when U-Boot starts.
 
    For example, on iMX8::
@@ -79,7 +79,7 @@
        };
 
 2. If your serial port requires a particular pinmux configuration, you may need
-   a pinctrl driver. This needs to have a u-boot,pre-reloc tag also. Take care
+   a pinctrl driver. This needs to have a u-boot,dm-pre-reloc tag also. Take care
    that any subnodes have the same tag, if they are needed to make the correct
    pinctrl available.