msm: acpuclock-krait: Make ACPU levels reference L2 levels by index

By referencing L2 frequency table rows by indexes rather than pointers,
we allow for future enhancements which will allocate the L2 frequency
table at runtime.

Change-Id: Icc62962d5d7b27dbf39008bc6eff83c4ee8951c4
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>