ENGR00173869-3: i.mx6: add the cpu_is_mx6dl() support

ENGR00173869-3: i.mx6: add the cpu_is_mx6dl() support

In order to support one image for i.mx6q and i.mx6dl, we introduce
the below functions by diff the value reading from ANATOP ID register.
cpu_is_mx6q() and cpu_is_mx6dl()

The layout for the register defines:
               Major  Minor
i.MX6Q1.1:       6300     01
i.MX6Q1.0:       6300     00
i.MX6DL1.0:      6100     00

For the common bits shared across all i.mx6 ports, we can use:
cpu_is_mx6() for it.

Signed-off-by: Jason Liu <r64343@freescale.com>