TF-A AMU extension: fix detection of group 1 counters.

This patch fixes the bug when AMUv1 group1 counters was
always assumed being implemented without checking for its
presence which was causing exception otherwise.
The AMU extension code was also modified as listed below:
- Added detection of AMUv1 for ARMv8.6
- 'PLAT_AMU_GROUP1_NR_COUNTERS' build option is removed and
number of group1 counters 'AMU_GROUP1_NR_COUNTERS' is now
calculated based on 'AMU_GROUP1_COUNTERS_MASK' value
- Added bit fields definitions and access functions for
AMCFGR_EL0/AMCFGR and AMCGCR_EL0/AMCGCR registers
- Unification of amu.c Aarch64 and Aarch32 source files
- Bug fixes and TF-A coding style compliant changes.

Change-Id: I14e407be62c3026ebc674ec7045e240ccb71e1fb
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>
diff --git a/docs/getting_started/porting-guide.rst b/docs/getting_started/porting-guide.rst
index 6b8bbc6..f331616 100644
--- a/docs/getting_started/porting-guide.rst
+++ b/docs/getting_started/porting-guide.rst
@@ -562,21 +562,14 @@
    doesn't print anything to the console. If ``PLAT_LOG_LEVEL_ASSERT`` isn't
    defined, it defaults to ``LOG_LEVEL``.
 
-If the platform port uses the Activity Monitor Unit, the following constants
+If the platform port uses the Activity Monitor Unit, the following constant
 may be defined:
 
 -  **PLAT_AMU_GROUP1_COUNTERS_MASK**
    This mask reflects the set of group counters that should be enabled.  The
    maximum number of group 1 counters supported by AMUv1 is 16 so the mask
    can be at most 0xffff. If the platform does not define this mask, no group 1
-   counters are enabled. If the platform defines this mask, the following
-   constant needs to also be defined.
-
--  **PLAT_AMU_GROUP1_NR_COUNTERS**
-   This value is used to allocate an array to save and restore the counters
-   specified by ``PLAT_AMU_GROUP1_COUNTERS_MASK`` on CPU suspend.
-   This value should be equal to the highest bit position set in the
-   mask, plus 1.  The maximum number of group 1 counters in AMUv1 is 16.
+   counters are enabled.
 
 File : plat_macros.S [mandatory]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~