ENGR00274009 ARM: imx: gpt clk on i.mx6sl can NOT source from OSC

ENGR00274009 ARM: imx: gpt clk on i.mx6sl can NOT source from OSC

There is no OSC clock source option for gpt on i.mx6sl, it
can only source from perclk, and perclk can be from OSC.

As perclk is clock source of many low speed devices, set
it to be from OSC, this can avoid freq varying when bus
clk(ipg) is scaled.

Signed-off-by: Anson Huang <b20788@freescale.com>