of/fdt: Remove of_scan_flat_dt() usage for __fdt_scan_reserved_mem()

Use of the of_scan_flat_dt() function predates libfdt and is discouraged
as libfdt provides a nicer set of APIs. Rework __fdt_scan_reserved_mem()
to be called directly and use libfdt.

Cc: Frank Rowand <frowand.list@gmail.com>
Link: https://lore.kernel.org/r/20211029183615.2721777-1-robh@kernel.org/


Signed-off-by: Rob Herring <robh@kernel.org>