arm64: Simplify checking for populated DT

Use of the of_scan_flat_dt() function predates libfdt and is discouraged
as libfdt provides a nicer set of APIs. Rework dt_scan_depth1_nodes to
use libfdt calls directly, and rename it to dt_is_stub() to reflect
exactly what it checking.

Cc: Will Deacon <will@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20211029144055.2365814-1-robh@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>