encore: clean up overclocking config #ifdefs

* Ensure upstream defaults are used when building for non-encore boards
* Don't make DSP OPP4 available on encore with stock clocks
encore: rework MPU and L3 overclocking

The 2.6.32 kernel previously used on encore used L3 OPPs of 83 and 166
MHz for the OMAP3621, whereas we currently use 100 and 200 MHz (the
OMAP3630 levels).  As some users have reported stability problems at the
higher L3 clocks, use the lower L3 OPPs for all the existing MPU clock
options.

For users lucky enough to have more capable chips, introduce new _HIL3
MPU options at 1000 MHz and 1100 MHz, which give the user the OMAP3630
L3 clocks as well as the specified overclocks.

To make the #ifdefs make more sense, introduce new hidden config entries
ENCORE_L3_UNDERCLOCK, ENCORE_MPU_ENABLE_{1100MHZ,1200MHZ} and
ENCORE_MPU_DISABLE_1000MHZ which do exactly what their names suggest and
nothing more.  The user-visible CONFIG_ENCORE_MPU_* options now select
the appropriate hidden config entries.
encore defconfigs: use 1200 MHz instead of 1100 MHz as default overclock

With the change to the default L3 clocks, more devices should be able to
handle the 1200 MHz MPU clock (and users may want the higher MPU clock
speed to mitigate some of the performance hit of dialing back the L3
clock).