Platform: Add support for new SOCs: nRF5340 and nRF9160

Supporting most configurations.

This patch targets the nRF9160 and the nRF5340 application core.

The nRF5340 is a dual-core SOC (application core and network core). The
nRF5340 application core is a Cortex-M33F with TrustZone-M.

The nRF9160 is an SOC with LTE-M/NB-IOT/GPS connectivity. The nRF9160 has
a Cortex-M33F with TrustZone-M.

Common code between the two platforms has been placed into a common
directory.

The platforms do not use the tfm_common_s.ld linker script since the nRF
IDAU needs some alterations to the placement of the veneers.

The common directory contains parts of the nrfx driver framework,
copied from https://github.com/NordicSemiconductor/nrfx version 2.3.0
at commit 6c31cb7d0e4038afa1efc9bfed40206adfd7b83e.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>
Change-Id: I15b932f444aa9adf2ef2489e9ecf036c64c851a7
diff --git a/platform/ext/index.rst b/platform/ext/index.rst
index 60422e9..d1ce4ef 100644
--- a/platform/ext/index.rst
+++ b/platform/ext/index.rst
@@ -38,6 +38,13 @@
 
     target/stm/**
 
+.. toctree::
+    :maxdepth: 4
+    :caption: Nordic
+    :glob:
+
+    target/nordic_nrf/**
+
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