Add lib_sw_pll to the IO framework?

[mbanth]

XMOS has a source code library, lib_sw_pll, that provides a software phased locked loop capability for tracking a clock. It is particularly useful for adaptive clocking of certain streaming interfaces such as I2S and USB. This source code library is currently internal to XMOS, but could be made public. Should lib_sw_pll become a part of the IO framework?

[keithm-xmos]

The plan is to break up fwk_io in order to return to the multi-repo approach for the peripheral libraries currently in the framework: I2C, SPI, UART & I2S.

My recommendation is that we do not add any libraries to the framework.