xen/arm: mm: clean up code in setup_pagetables

xen/arm: mm: clean up code in setup_pagetables

The base of address for the relocated xen needs to be mapped
at the same virtual address (BOOT_RELOC_VIRT_START) in both
the boot and runtime page tables. So we can merge the two pieces
of code into one code block.

Also no need to use write_pte when mapping BOOT_RELOC_VIRT_START
in xen_second, because CPU0 is using boot page tables.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>