Add support for setting SMB clock output to arbitrary frequency

[adamgreig]

• Refactored si5338.c a little to reuse the multisynth related functions for both CLK3A (the SMB output and extension board clock) and the original CLK1/2 going to the LMS for sampling.
• Added new functions to si5338.c to get/set rational/integer frequencies for CLK3A
• Expose those functions in bladerf.c and export to the libbladeRF.h API
• Extend "mimo master" command in bladeRF-cli to support an optional frequency

I've tested locally against an oscilloscope and this works as expected for me, though I can only measure up to ~100MHz output clocks directly.

Very open to suggestions and changes - this is mostly based on a quick chat on IRC after I implemented setting the SMB to 10MHz in a quick C program to solve my particular need.

[bpadalino]

Maximum output frequency for driving CMOS with the si5338 is 200MHz so it should be limited to that.

Reviewing more.

[jynik]

@bpadalino - It looks like this was merged. Can I close this?

[bpadalino]

@jynik - Committed in 01940e6 and not if I can close it first!