SPM: Add support for watermarking SPM stack

The IPC backend in a build without CONFIG_USE_TRUSTZONE set
allocates a dedicated SPM stack, rather than SPM using the
stack allocated to a partition.

Update the stack watermarking code (CONFIG_TFM_STACK_WATERMARKS)
so that this dedicated stack, if created, will also be watermarked
and the usage reported alongside the partition stack usage.

Change-Id: I51aeb98d85e4284a10b955bb0da7f96ee948e578
Signed-off-by: Chris Brand <chris.brand@cypress.com>
diff --git a/docs/configuration/index.rst b/docs/configuration/index.rst
index 27fdac9..6783685 100644
--- a/docs/configuration/index.rst
+++ b/docs/configuration/index.rst
@@ -279,6 +279,8 @@
 +----------------------------------------+-----------+-------------+
 |TFM_SPM_LOG_LEVEL                       | Build     |   1         |
 +----------------------------------------+-----------+-------------+
+|CONFIG_TFM_STACK_WATERMARKS             | Build     |   OFF       |
++----------------------------------------+-----------+-------------+
 |CONFIG_TFM_CONN_HANDLE_MAX_NUM          | Component |   8         |
 +----------------------------------------+-----------+-------------+
 |CONFIG_TFM_DOORBELL_API                 | Component |   0         |