arm64: make dt_scan_depth1_nodes more readable

Improve the readability of dt_scan_depth1_nodes by removing the nested
conditionals.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>