Cache maintenance operations require reference to self

[ryan-summers]

Various cache maintenance-related operations require a self reference, which means that it is not possible for a HAL or other peripherals to clean/invalidate and/or perform other cache related operations without owning the SCB.

In the context of DMA, this can be a stumbling block, as DCACHE needs to be cleaned and invalidated before and after transfers (if it is enabled).

Is it possible to refactor the cache maintenance functions to not require a self reference so that they can be called statically as cortex_m::peripherals::scb::invalidate_dcache_by_address()?

[ryan-summers]

Related to #238