dts: boards: Define dts aliases at soc level for i.mx 6/7 socs

Defines device tree aliases for on-chip peripherals at the soc level
instead of the board level for all i.mx 6/7 socs. The eliminates some
duplicate code in the board level device trees, and will allow drivers
to use device-tree generated macros directly instead of through dts
fixups.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>