ARC: [haps] Add Virtio support

As a preparation for QEMU usage for ARC let's add basic Virtio-MMIO
peripherals support for the platform we're going to use.

For now we add 5 Virtio slots in .dts and enable block and network devices
via Virtio-MMIO.

Note even though typically Virtio register set fits in 0x200 bytes
we "allocate" here 0x2000 so that it matches ARC's default 8KiB page size
and so remapping of that area is done clearly.

We also enable DEVTMPFS automount for more convenient use
of external root file-stystem. Before that we used to use built-in
Initramfs which didn't automount DEVTMPFS anyways so we didn't need
that option, while now it starts making sense.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>