OMAP3 clock: only unlock SDRC DLL if SDRC clk < 83MHz

According to the 34xx TRM Rev. K section 11.2.4.4.11.1 "Purpose of the
DLL/CDL Module," the SDRC delay-locked-loop can be locked at any SDRC
clock frequency from 83MHz to 166MHz.  CDP code unconditionally
unlocked the DLL whenever shifting to a lower SDRC speed, but this
seems unnecessary and error-prone, as the DLL is no longer able to
compensate for process, voltage, and temperature variations.  Instead,
only unlock the DLL when the SDRC clock rate would be less than 83MHz.

Signed-off-by: Paul Walmsley <paul@pwsan.com>