Changed the rp2040/rp235x VCO max to 1600 MHz

[jonathanherbstgrapple]

The rp code allows VCO frequency up to 1800 MHz, which is outside of the range specified by the datasheets (750 - 1600 MHz). This pr changes that to 1600 Mhz.

Changes:

• Added variables for VCO min and max.
• Modified code to use the VCO_MIN and VCO_MAX variables.

Testing:

• I ran overclock and overclock_manual examples on a rpi pico, they both clock to 200 MHz.
• I couldn't figure out how to run the unit tests, but I assume they will get run by the ci?

Related Issues:

• Should RP2040/2350 PLL VCO max be 1800 MHz? #4850