AMU: Add plat interface to select which group 1 counters to enable

AMU: Add plat interface to select which group 1 counters to enable

A new platform macro `PLAT_AMU_GROUP1_COUNTERS_MASK` controls which
group 1 counters 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.

A related platform macro `PLAT_AMU_GROUP1_NR_COUNTERS` is used by
generic code to allocate an array to save and restore the counters on
CPU suspend.

Change-Id: I6d135badf4846292de931a43bb563077f42bb47b
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>