davinci: display correct clock info

Move the clock-rate dumping code into the cpu/.../davinci area
where it should have been, enabled by CONFIG_DISPLAY_CPUINFO,
updating the format and showing the DSP clock (where relevant).

Switch boards to use the cpuinfo() hook for this stuff.

Remove a few now-obsolete PLL #defines.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>