imx6: increase coherent pool to 1M

this allows some dvb sticks (at least dvb_usb_it913x) to load
their firmware

[    5.240055] ERROR: 256 KiB atomic DMA coherent pool is too small!
[    5.240055] Please increase it with coherent_pool= kernel parameter!
[    5.241779] dvb_usb_it913x: probe of 1-1:1.0 failed with error -12