icache: Add ICACHE management

[astapleton]

Add management functions for the ICACHE instruction cache peripheral.

[astapleton]

@ryan-summers Ok, I've reworked this API significantly. It removes a lot of the edge cases for behavior when the ICACHE is disabled vs enabled or what mode it is in by having the ICache struct only be valid while the peripheral is enabled. Enabling it in either mode gives you the ICache struct, and disabling the peripheral drops it returning the peripheral struct ICACHE that can then be re-enabled. I also added interrupt enable/disable and servicing methods.

[ryan-summers]

That looks much nicer from a design perspective! :)