ENGR00300439-5 can: flexcan: parse stop mode control bits from device tree

ENGR00300439-5 can: flexcan: parse stop mode control bits from device tree

Starting from IMX6, the flexcan stop mode control bits is SoC specific,
move it out of IP driver and parse it from devicetree.
It's good from maintain perspective and can avoid adding too many SoC
specifi bits in driver but with no IP changes when the IMX SoC series
keep growing.

Signed-off-by: Dong Aisheng <b29396@freescale.com>