ewaol-bsp/xen-devicetree: Use the correct variable to set Dom0 memory size

We should use EWAOL_CONTROL_VM_MEMORY_SIZE instead of EWAOL_HOST_MEMORY_SIZE to
set XEN_DEVICETREE_DOM0_MEM.

This is a left over from:
    ff4907b4 doc,ewaol-[bsp,config,distro,tests]: Update EWAOL nomenclature

Issue-Id: SCM-3910
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Change-Id: I6578d77774878a754d88670721615776998d6592