ARM: Fix dma_dcache_sync() for early allocated memory.
Drivers can request DMA to buffers that are not in memory represented
in the vm page arrays. Because of this, store KVA of already mapped
buffer to synclist and use it in dma_dcache_sync().

Reviewed by: jah
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D4120