drivers: base: expose amount allocated per block

Add a new sysfs node called allocated_bytes to each memory block which
reports the amount of memory currently allocated from that block.

Userspace can then use the allocated_byes node to better evaluate the
potential cost of offlining an individual block since all the allocated
memory in the block will need to be migrated when the block is offlined.

Change-Id: I520de2dd56e4562a65c48e38edb1236a8cbce5fd
Signed-off-by: Liam Mark <lmark@codeaurora.org>