of: reserved-memory: Fix using wrong number of cells to get property 'alignment'

According to DT spec, size of property 'alignment' is based on parent
node’s #size-cells property.

But __reserved_mem_alloc_size() wrongly uses @dt_root_addr_cells to get
the property obviously.

Fix by using @dt_root_size_cells instead of @dt_root_addr_cells.

Fixes: 3f0c8206 ("drivers: of: add initialization code for dynamic reserved memory")
Cc: stable@vger.kernel.org
Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Link: https://lore.kernel.org/r/20250109-of_core_fix-v4-9-db8a72415b8c@quicinc.com


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