OMAP1 clock: convert test in disable_unused() to use ENABLE_ON_INIT

mach-omap1/clock.c:omap1_clk_disable_unused() contains a test that
assumes that the clock structures are available in the file's
namespace.  After a following patch, this will no longer be the case.
So we need to reimplement that test.  It turns out that we already
have a facility in the clock framework to handle this case - the
ENABLE_ON_INIT flag - used on OMAP2/3.  Remove the offending test and
mark the clocks that it was intended to catch as ENABLE_ON_INIT.

Signed-off-by: Paul Walmsley <paul@pwsan.com>