Create a common i.MX53 config and use it with the two existing i.MX53
boards.

This is just intended to split the common config entries out, further
cleanup is expected.

Reviewed by:	ian@, rpaulo@ (earlier version)
Differential Revision: https://reviews.freebsd.org/D731


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271220 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f