ENGR00318063-14: ARM: imx6sx: set pll2_bus as the default parent of QSPI clock

ENGR00318063-14: ARM: imx6sx: set pll2_bus as the default parent of QSPI clock

We already know that pll2_bus is the best parent of QSPI clock.  Let's
set it up in clock initialization as the default, so that we can save
those implicit parent switching and rate changing calls from clock
framework.

shawn.guo: cherry-pick commit ecef6a715f1a from imx_3.10.y

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