arm64: idreg-override: use early FDT mapping in ID map

Instead of calling into the kernel to map the FDT into the kernel page
tables before even calling start_kernel(), let's switch to the initial,
temporary mapping of the device tree that has been added to the ID map.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20220624150651.1358849-16-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>