ARM: dts: imx6: make gpt per clock can be from OSC

Original gpt per clk parent is from ipg_per clk which
may be scaled when system enter low bus mode, as ipg
clk will be lower in low bus mode, to keep system clk
NOT drift, select gpt per clk parent from OSC which
is at fixed freq always.

On i.mx6qdl, add a osc_per clk source for i.mx6q
TO > 1.0 and all i.MX6dl SoC.

On i.mx6sx, just make gpt per clk from OSC.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>