ARM: imx: add a common function to initialize revision from anatop

The patch creates a common function imx_init_revision_from_anatop() by
merging imx6q_init_revision() and imx_anatop_get_digprog(), so that any
SoC that encodes revision info in anatop can use it to initialize
revision.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>