doc: uefi: fix links

Fix a couple of links so that they are rendered correctly with sphinx.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/develop/uefi/fwu_updates.rst b/doc/develop/uefi/fwu_updates.rst
index 72c850a..e4709d8 100644
--- a/doc/develop/uefi/fwu_updates.rst
+++ b/doc/develop/uefi/fwu_updates.rst
@@ -27,7 +27,8 @@
 media, and its partitioning method. Details of the storage device
 containing the FWU metadata partitions are specified through a U-Boot
 specific device tree property `fwu-mdata-store`. Please refer to
-U-Boot `doc <doc/device-tree-bindings/firmware/fwu-mdata-gpt.yaml>`__
+U-Boot :download:`fwu-mdata-gpt.yaml
+</device-tree-bindings/firmware/fwu-mdata-gpt.yaml>`
 for the device tree bindings.
 
 Enabling the FWU Multi Bank Update feature