xen: arm: flush dcache while preparing the device tree for Dom0

xen: arm: flush dcache while preparing the device tree for Dom0

Without flushing dcache the hypervisor couldn't copy the device tree
correctly when booting the kernel dom0 Image (memory with device tree
is corrupted). As the result - when we try to load the kernel dom0
Image - dom0 hungs frequently. This issue is not reproduced with the
kernel dom0 zImage because the zImage decompressor code flushes all
dcache before starting the decompressed kernel Image. When the
hypervisor loads the kernel image or initrd, this memory region
isn't corrupted because the hypervisor code flushes the dcache.

Signed-off-by: Oleksandr Dmytryshyn <oleksandr.dmytryshyn@globallogic.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall<julien.grall@linaro.org>