Update clock configuration

[jmadden173]

The old firmware needs its clock configuration fixed for low power measurements. Plan on copying over the configuration from there. See jlab-sensing/ENTS-node-firmware#268

[jmadden173]

Running list of things I think need to be fixed:

• RTC runs on a 32.768k external clock
• Want to clock down to 4MHz -> Lower power consumption
• Update SYSCLK to use LSE -> Lower power consumption

Other notes:

• Peripherals buses use PCLK when in the old firmware it relies on SYSCLK

[jmadden173]

Ensure that the I2C clock is correct, see #3 (comment)