Blackfin arch: Extend sram malloc to handle L2 SRAM.

Extend system call to alloc L2 SRAM in application.
Automatically move following sections to L2 SRAM:
1. kernel built-in l2 attribute section
2. kernel module l2 attribute section
3. elf-fdpic application l2 attribute section

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>