ARM: imx: add gpt_3m clk for i.mx6qdl

Add gpt_3m clock for i.mx6qdl, as gpt can source clock
from OSC, some i.MX6 series SOCs has fixed divider of
8 for gpt clock, so here add a fix clk of gpt_3m.

i.MX6Q TO1.0 has no gpt_3m option, so force it to be
from ipg_per.

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