omap2 clock: get rid of sleep_ck

According to the 2420 TRM rev J, SLEEP_CLK doesn't have any registers
or bits associated with it, so it's outside software control.  It's
also a leaf clock - no software-controllable clocks use it as a
parent.  There's nothing we can do with it in software.  So, drop it.

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