FROMLIST: memblock: support memsize reusable to consider as reusable

Sometimes we'd like to include some memory region as reusable even
though it is actually not CMA type. This could be applied to cleancache
memory. If we want the cleancache memory to be considered as reusable,
add reusable in its device tree description, then memsize will say it is
reusuable and this commit will add its size to the system total memory
size.

Bug: 340432773
Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com>
Link: https://lore.kernel.org/linux-mm/20240521023957.2587005-11-jaewon31.kim@samsung.com/
Change-Id: I0bf8006a397ae7a7f136281c9712dde9dffe38b2