ARM: at91: pm: cpu_idle: switch DDR to power-down mode

On some DDR controllers, compatible with the sama5d3 one,
the sequence to enter/exit/re-enter the self-refresh mode adds
more constrains than what is currently written in the at91_idle
driver. An actual access to the DDR chip is needed between exit
and re-enter of this mode which is somehow difficult to implement.
This sequence can completely hang the SoC. It is particularly
experienced on parts which embed a L2 cache if the code run
between IDLE calls fits in it...

Moreover, as the intention is to enter and exit pretty rapidly
from IDLE, the power-down mode is a good candidate.

So now we use power-down instead of self-refresh. As we can
simplify the code for sama5d3 compatible DDR controllers,
we instantiate a new sama5d3_ddr_standby() function.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: <stable@vger.kernel.org> # v4.1+
Fixes: 017b5522 ("ARM: at91: Add new binding for sama5d3-ddramc")
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>