refactor(amu): conditionally compile auxiliary counter support

This change reduces preprocessor dependencies on the
`AMU_GROUP1_NR_COUNTERS` and `AMU_GROUP1_COUNTERS_MASK` definitions, as
these values will eventually be discovered dynamically.

In their stead, we introduce the `ENABLE_AMU_AUXILIARY_COUNTERS` build
option, which will enable support for dynamically detecting and
enabling auxiliary AMU counters.

This substantially reduces the amount of memory used by platforms that
know ahead of time that they do not have any auxiliary AMU counters.

Change-Id: I3d998aff44ed5489af4857e337e97634d06e3ea1
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 1259881..f9999e1 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -220,6 +220,10 @@
    v8.2 implementations also implement an AMU and this option can be used to
    enable this feature on those systems as well. Default is 0.
 
+-  ``ENABLE_AMU_AUXILIARY_COUNTERS``: Enables support for AMU auxiliary counters
+   (also known as group 1 counters). These are implementation-defined counters,
+   and as such require additional platform configuration. Default is 0.
+
 -  ``ENABLE_ASSERTIONS``: This option controls whether or not calls to ``assert()``
    are compiled out. For debug builds, this option defaults to 1, and calls to
    ``assert()`` are left in place. For release builds, this option defaults to 0
@@ -918,4 +922,3 @@
 
 .. _DEN0115: https://developer.arm.com/docs/den0115/latest
 .. _PSA FW update specification: https://developer.arm.com/documentation/den0118/a/
-