Husky: allow target clock below 5 MHz

[jpcrypt]

Although Husky's main PLL allows generating clocks down to 500 KHz, currently Husky can only go as low as 5 MHz because this clock feeds other PLLs that are used for:

• glitching
• UART and trace triggering
• logic analyzer

However if we disallow using the target clock as a source clock for those functions, then we should be able to run Husky below 5 MHz (to be clear this is not currently possible).