Help: ICE_FLAGS_MUXCLK=S

IOC code allowing choice of 6 on-board clock sources

Loads IOC code allowing muxed clock operation.  Both modules use the same 
clock.  Outputs always use MUXCLK.  There 6 possible sources for the muxed 
clock signal:

   s=I Internal clock = 40MHz/N where N is an integer (1-4096)
   s=P Programmable Clock on series 4+ cards
   s=A Module A input clock (or s=1)
   s=B Module B input clock (or s=2)
   s=C Alternate Crystal on series 3+ cards
   s=D Alternate Crystal CCLK/N where N is an integer (1-64), (1-16) on series 3
   s=X External clock on series 3+ cards
   s=N No MUXCLK

The A2DR9 modules have their own programmable clocks.  When specifying MUXCLK=P
or PREFX with these modules, the chip and reference on the modules are used.