staging: fsl-mc: move mc-cmd.h contents in the public header

mc-cmd.h contains some low level functions used to encode and decode
commands to the MC. They are used by the drivers so move them to the
public headers and get rid of the mc-cmd.h header.

Signed-off-by: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>