arm64: setup: drop early FDT pointer helpers

We no longer need to call into the kernel to map the FDT before calling
into the kernel so let's drop the helpers we added for this.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20220624150651.1358849-22-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>