Add i.MX 6 IPU driver and enable it in IMX6 config

Current functionality is somewhat limited: driver assumes that there
is only one active IPU unit (IPU1) and that video output is DI0 and
video mode is 1024x768. For more advanced functionality driver requires
proper clock management which is work in progress. At the moment driver
assumes that pixel clock is configured by u-boot for 1026x768 mode.

Reviewed by: andrew, ian, mmel
Differential Revision: https://reviews.freebsd.org/D4168