arm64: kexec: Copy and install a copy of the linear-map

To perform the kexec relocations with the MMU enabled, we need a copy
of the linear map.

Create one, and install it from the relocation code. This has to be done
from the assembly code as it will be idmapped with TTBR0. The kernel
runs in TTRB1, so can't use the break-before-make sequence on the mapping
it is executing from.

The makes no difference yet as the relocation code runs with the MMU
disabled.

Signed-off-by: James Morse <james.morse@arm.com>