Invert the logic in imx8mq_cpu_revision() and convert it to use early
return. This is done in perparation for more corner case logic added
in the following commits. No functional change intended.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>