MLK-11391 ARM: imx: make sure SNVS clk is enabled during suspend

As SNVS clk may be disabled in kernel to save power(~1mA),
but during suspend/resume, we have to access SNVS register
to do MMDC retention and power down SoC etc.., need to
make sure SNVS clk is enabled before accessing its register.

Signed-off-by: Anson Huang <b20788@freescale.com>