ARM: Cortex-A9: Enable dynamic clock gating

ARM: Cortex-A9: Enable dynamic clock gating

Enable dynamic high level clock gating for Cortex-A9 CPUs, as
described in 2.3.3 "Dynamic high level clock gating" of the
Cortex-A9 TRM.  This may cut the clock of the integer core,
system control block, and Data Engine in certain conditions.

Add ARM errata 720791 to avoid corrupting the Jazelle
instruction stream on earlier Cortex-A9 revisions.

Change-Id: I48e51d907e593f26982ea91b0a811553f68e3c86
Signed-off-by: Todd Poynor <toddpoynor@google.com>