[iMX6] Add compatibility string for GPT timer on i.MX6 Dual

Up until r295436 GPT timer in i.MX6 Dual dts used the same compatiblity
string as i.MX6 Quad. After the sync up with Linux in r295436, GPT timer
stopped getting attached on the i.MX6 Dual

MFC after: 3 days