ARM: mx3: Let mx31 and mx35 enter in LPM mode in WFI

The LPM field of register CCMR is used to select the mode that the processor will run
when it goes to WFI.

When mx31 enters in WFI mode the LPM field is at its reset value of 0,
which configures the mx31 to enter in "wait mode".

On mx35, the LPM field on mx35 is also at 0 after reset, which corresponds
to "run mode" instead of "wait mode".

Instead of relying on the reset value of LPM to set the low power mode for
WFI, configure mx31 and mx35 to run in "wait mode"

Reported-by: Benoit Thebaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>