ARM: imx7d: correct chip version information

The commond 'cat /sys/devices/soc0/revision' can
show correct soc version information.

"unknow revision" message in imx_print_silicon_rev()
will never work for digprog.

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>