xen/arm64: do not (incorrectly) limit size of xenheap

xen/arm64: do not (incorrectly) limit size of xenheap

The commit 88e3ed61642bb393458acc7a9bd2f96edc337190 "x86/NUMA: make
init_node_heap() respect Xen heap limit" breaks boot on the arm64 board
X-Gene.

The xenheap bits variable is used to know the last RAM MFN always mapped
in Xen virtual memory. If the value is 0, it means that all the memory is
always mapped in Xen virtual memory.

On X-gene the RAM bank resides above 128GB and last xenheap MFN is
0x4400000. With the new way to calculate the number of bits, xenheap_bits
will be equal to 38 bits. This will result to hide all the RAM and the
impossibility to allocate xenheap memory.

Given that aarch64 have always all the memory mapped in Xen virtual
memory, it's not necessary to call xenheap_max_mfn which set the number
of bits.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>