The DCE bit in the MSR controls whether or not the cache is enabled. When
disabling caches you must ensure that all the prior writes within the cacheable range
are completed in external memory before reading back over M_AXI_DP. This can be done by writing to a semaphore immediately before
turning off caches, and then in a loop poll until it has been written. The contents of
the cache are preserved when the cache is disabled.