ARM: imx6sx: Set PLL2 as parent of QSPI clocks

ARM: imx6sx: Set PLL2 as parent of QSPI clocks

The default qspi2_clk_sel field of register CCM_CS2CDR contains '110' which is
marked as 'reserved', so we can't rely on the default value.

Provide a proper parent for QSPI clocks to avoid a kernel oops:

[    1.037920] Division by zero in kernel.
[    1.041807] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.18.0-rc7-next-20141204-00002-g5aa23e1 #2143
[    1.050967] Hardware name: Freescale i.MX6 SoloX (Device Tree)
[    1.056853] Backtrace:
[    1.059360] [<80011ea0>] (dump_backtrace) from [<8001203c>] (show_stack+0x18/0x1c)
[    1.066982]  r6:00000000 r5:00000000 r4:00000000 r3:00000000
[    1.072754] [<80012024>] (show_stack) from [<806b7100>] (dump_stack+0x88/0xa4)
[    1.080038] [<806b7078>] (dump_stack) from [<80011d20>] (__div0+0x18/0x20)
[    1.086958]  r5:be018500 r4:be017c00
[    1.090600] [<80011d08>] (__div0) from [<802aa418>] (Ldiv0+0x8/0x10)
[    1.097012] [<80504fbc>] (clk_divider_set_rate) from [<80503ddc>] (clk_change_rate+0x14c/0x17c)
[    1.105759]  r7:00000000 r6:00000000 r5:be018500 r4:00000000
[    1.111516] [<80503c90>] (clk_change_rate) from [<80503ea0>] (clk_set_rate+0x94/0x98)
[    1.119391]  r8:be7e0368 r7:00000000 r6:be11a000 r5:be018500 r4:00000000 r3:00000000
[    1.127290] [<80503e0c>] (clk_set_rate) from [<80410558>] (fsl_qspi_probe+0x23c/0x75c)
[    1.135260]  r5:be11a010 r4:be350010
[    1.138900] [<8041031c>] (fsl_qspi_probe) from [<80385a18>] (platform_drv_probe+0x50/0xac)

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>