imx-common: timer: support i.MX6DQPlus

To i.MX6DQPlus, osc can be choosed as the source of gpt, so
add i.MX6DQPlus support in gpt_has_clk_source_osc.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>