ARM: imx6sx: add BYPASS support for PLL clocks

This is the same change for imx6sx clock driver as "ARM: imx6q: add BYPASS
support for PLL clocks" for imx6q.  The difference is that only anaclk1
is available on imx6sx.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>