docs(ethos-n): update porting-guide.rst for NPU

Add some missing configuration that must be done for supporting NPU on
other platforms.

Signed-off-by: Rob Hughes <robert.hughes@arm.com>
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>
Change-Id: Ic505ea60f73b970d0d7ded101830eb2ce8c7ab64
diff --git a/docs/plat/arm/arm-build-options.rst b/docs/plat/arm/arm-build-options.rst
index 83212c6..e7e7ee7 100644
--- a/docs/plat/arm/arm-build-options.rst
+++ b/docs/plat/arm/arm-build-options.rst
@@ -105,8 +105,9 @@
    Arm® Ethos™-N NPU. Requires ``ARM_ETHOSN_NPU_DRIVER`` and
    ``TRUSTED_BOARD_BOOT`` to be enabled.
 
--  ``ARM_ETHOSN_NPU_FW``: location of the NPU firmware binary (ethosn.bin).
-   This firmware image will be included in the FIP and loaded at runtime.
+-  ``ARM_ETHOSN_NPU_FW``: location of the NPU firmware binary
+   (```ethosn.bin```). This firmware image will be included in the FIP and
+   loaded at runtime.
 
 -  ``ARM_SPMC_MANIFEST_DTS`` : path to an alternate manifest file used as the
    SPMC Core manifest. Valid when ``SPD=spmd`` is selected.