omap2 clock: convert PARENT_CONTROLS_CLOCK into a clock flag

The clock framework currently uses a magic value in struct clk
.enable_bit to indicate that the clock's parent controls enabling and
disabling the clock.  There's no need to use this type of in-band
special value when there's already a good means to indicate binary
clock parameters: the clock flags field.  This patch converts the
existing PARENT_CONTROLS_CLOCK code to use a clock flag.

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