Blackfin arch: implement a basic /proc/sram file for L1 allocation visibility

implement a basic /proc/sram file for L1 allocation visibility until we can
rewrite the entire L1 allocator (which would include a proper mechanism)

Signed-off-by: Mike Frysinger <michael.frysinger@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>