MAM: Provide ability to zero memory regions

[imphil]

This is a request to specify the ability for the Memory Access Module to fill a region of memory with 0s.

Use case: when loading an application binary we currently transfer it in full, including all sections which are zero-filled, e.g. the .bss segment. Instead of transferring this segment we could simply instruct MAM to zero this part of the memory. This saves significant amounts of off-chip bandwidth, e.g. when initializing a memory over UART, or in simulation.