spi: reenable sync SPI transfers

one source of excess latency in hm2_spi / hal_spidev is latency due
to async transfers.  Make the __spi_sync primitive actually synchronous,
rather than building on an asynchronous primitive.
spidev: Avoid runtime memory allocations

allocating bounce and k_xfers at every iteration was a source of
excess delays.
spidev: actually use synchronous transfers

this reduces a source of latency
spidev-s3c64xx: allocate dma channel at startup

it appears that acquire_dma introduces intolerable delays; the system
is fine with the dma channnels allocated all the time.