Document: dt: fsl: snvs: change support syscon

snvs actually is multi fucntion driver.
Change to use syscon to access register.
Change snvs parent interrupt to option because single function
may have seperated irq number.

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>