juno/juno_clock.h: Add indices for the different clocks

This patch adds enumeration of the indices for the different
clocks available on the Juno platform.

Change-Id: Iea0f7272d688ae2141f3ea62d817db058bc78b32
Signed-off-by: Elieva Pignat <Elieva.Pignat@arm.com>
Signed-off-by: Jim Quigley <jim.quigley@arm.com>
Signed-off-by: Raphael Gault <raphael.gault@arm.com>
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>