MFC r309999, r310012

r309999:
[iMX6] Fix platform compatibility string for i.MX6 Dual

i.MX6 Dual boot was broken since r308533 because ofw_bus_node_is_compatible
is more strict than fdt_is_compatible and does not accept partial matches

r310012:
[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