decode: Add comments for instruction_valid and instruction_execute signals

Signed-off-by: Joel Fernandes <joelf@ti.com>
Add comments to how align_line works

Signed-off-by: Joel Fernandes <joelf@ti.com>
tb: Add comments for 128-bit memory loading

Signed-off-by: Joel Fernandes <joelf@ti.com>
Add more comments for decode/execute

Signed-off-by: Joel Fernandes <joelf@ti.com>