omap2: use symbolic constant to mark parent-controlled clocks in clock.[ch]

The current OMAP2 clock management code uses a magic number, 0xff, to
indicate that the clock's parent will handle enabling and disabling the
clock.  Move the magic number into a symbolic constant for readability.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>