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
2 files changed