OMAP2+: clock: reduce the amount of standard debugging while disabling unused clocks

Reduce the amount of debugging generated by default when unused clocks
are being disabled by the clock code.  The previous code would only
generate debug-level messages, but some people who wished to run
production kernels with debug-level messages enabled reported that the
large number of clock disable messages were slowing boot.  Now to
enable clock-by-clock disable messages, DEBUG needs to be defined in
mach-omap2/clock.c.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Tuukka Tikkanen <tuukka.tikkanen@nokia.com>
Cc: Tim Bird <tim.bird@am.sony.com>