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/lib/extensions/amu/aarch32/amu_helpers.S b/lib/extensions/amu/aarch32/amu_helpers.S
index d387341..8ac7678 100644
--- a/lib/extensions/amu/aarch32/amu_helpers.S
+++ b/lib/extensions/amu/aarch32/amu_helpers.S
@@ -84,6 +84,7 @@
 	bx		lr
 endfunc amu_group0_cnt_write_internal
 
+#if ENABLE_AMU_AUXILIARY_COUNTERS
 /*
  * uint64_t amu_group1_cnt_read_internal(int idx);
  *
@@ -267,3 +268,4 @@
 	stcopr	r1, AMEVTYPER1F /* index 15 */
 	bx	lr
 endfunc amu_group1_set_evtype_internal
+#endif