ENGR00179782: i.mx6: consolidate mx6q/dl_revision() support

ENGR00179782: i.mx6: consolidate mx6q/dl_revision() support

The idea is to get the soc silicon revision from DIGPROG register Of
ANATOP(USB_ANALOG_DIGPROG), which will make kernel code independent
with bootloader which need pass the system_rev by ATAG.

This patch also will print the chip name and revision when kernel boot
up since this information is important for customer to know.

on i.mx6q TO1.1, it will print as the following:

CPU identified as i.MX6Q, silicon rev 1.1

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