xen: arm: Write to the correct PT when mapping the DTB on boot on arm64

xen: arm: Write to the correct PT when mapping the DTB on boot on arm64

We currently get away with this because when debug=y and earlyprintk is enabled
the previous block of (conditional) code would have set this up. Historically we
mostly got away with it even without those options because the pre paging code
would normally (at least on h/w we test) leave x4 set to the paddr of
boot_second.

This latent bug has always been present but was exposed by ca59618967fe "xen:
arm: Handle 4K aligned hypervisor load address" (or one of the related patches)
since now x4 is quite likely to point to boot_third not boot_second.

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