mtd: spi-nor: fsl-qspi: support deep sleep mode for imx6 sx chip

i.mx6 sx support turn off fastmix and megamix power.
qpsi controller can be turned off and all status lost when suspend/resume.
add suspend/resume functions and reset qspi controller when resume.

Signed-off-by: Allen Xu <b45815@freescale.com>
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Acked-by: Huang Shijie <shijie.huang@intel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>