plat: imx7: refactor code for reuse

For the iMX7 SOCs, part of the code for platform
setup implementation can be reused and made
common for all these SoCs. This patch extracts
the common part for reuse.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Change-Id: I42fd4167e6903416df96a0159a046abf3896e878