bfin: pm: add deepsleep for bf60x

Add add deepsleep for bf60x.
1. Call DMC init functions to enter and exit DDR self refresh mode.
2. Wait till CGU PLL is locked after wake up and exit DDR self refresh mode.
3. Make asessembly function enter_deepsleep comply with C funtion ABI in
order to call other C functions.
4. Switch kernel stack by register EX_SCRATCH_REG.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>