ARC: [plat-hsdk]: allow to switch between AXI DMAC port configurations

We want to use DW AXI DMAC on HSDK board in our automated verification
to test cache & dma kernel code changes. This is perfect candidate
as we don't depend on any external peripherals like MMC card / USB
storage / etc.
To increase test coverage we want to test both options:
 * DW AXI DMAC is connected through IOC port & dma direct ops used
 * DW AXI DMAC is connected to DDR port & dma noncoherent ops used

Introduce 'arc_hsdk_axi_dmac_coherent' global variable which can be
modified by debugger (same way as we patch 'ioc_enable') to switch
between these options without recompiling the kernel.
Depend on this value we tweak memory bridge configuration and
"dma-coherent" DTS property of DW AXI DMAC.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>