imx: cleanup bootaux

Move i.MX6/7 bootaux code to imx_bootaux.c.
The i.MX6/7 has different src layout, so define M4 reg offset
to ease the cleanup. Redefine the M4 related BIT for share
common code.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>