Refactor hackrf_core.x in preparation for removal. (1/3)

[antoinevg]

This PR moves a number of items out of hackrf_core in preparation for removing it entirely:

• CPU clock logic => cpu_clock.x
• Clock generator logic => clock_gen.x
• Power logic => power.x
• LED logic => leds.x
• clkin_ctrl_set => clkin.x
• narrowband_filter_set => fpga.x
• trigger_enable => clock_gen.x

Follow-up PR's address the remaining items:

• Move hackrf_core.x driver instances to drivers.x (2/3) #1709
• Rename hackrf_core.x to pins.x (3/3) #1710