omap2 clock: replace omap2_get_crystal_rate() with clock-specific recalc code

The OMAP2 clock framework currently uses a nonstandard function to
assign the osc_ck and sys_ck rates, omap2_get_crystal_rate().  By
using custom recalc code for these clocks, we can get rid of it and
rely on the existing clock tree recalculation process.

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