xen/arm: domain_build: allocate lowmem for dom0 as much as possible

xen/arm: domain_build: allocate lowmem for dom0 as much as possible

On AArch64 SoCs, some IPs may only have the capability to access
32 bits address space. The physical memory assigned for Dom0 maybe
not in 4GB address space, then the IPs will not work properly.
So need to allocate memory under 4GB for Dom0.

There is no restriction that how much lowmem needs to be allocated for
Dom0 ,so allocate lowmem as much as possible for Dom0.

This patch does not affect 32-bit domain, because Variable "lowmem" is
set to true at the beginning. If failed to allocate bank0 under 4GB,
need to panic for 32-bit domain, because 32-bit domain requires bank0
be allocated under 4GB.

For 64-bit domain, set "lowmem" to false, and continue allocating
memory from above 4GB.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Julien Grall <julien.grall@arm.com>