BACKPORT: arm64: move kimage_vaddr to .rodata

Clean backport.

This datum is not referenced from .idmap.text: it does not need to be
mapped in idmap. Lets move it to .rodata as it is never written to after
early boot of the primary CPU.
(Maybe .data.ro_after_init would be cleaner though?)

Bug: 352610488
Change-Id: I83bc69358eca7eca1a5e9916bc6fe477f1c6ef96
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit 6cf9a2dce6bd10cf454cf6299c1c23182cb486e7)
Signed-off-by: Giuliano Procida <gprocida@google.com>