We need to make sure that CNTFRQ is initialized before corresponding
clocksource driver tries to use it, otherwise we'll end up crashing
due to division by zero.

We can't convert imx7_timer_init() to be an initcall since it is an
i.MX7 specific task, but CPU type information won't be availible until
after imx_init() gets executed at postcore_initcall() level. To solve
this move all of the necessary code to be a part of
imx7_cpu_lowlevel_init().

Note, that original code both hardcoded frequency value to 8MHz as
well as tried to write it to the first element of the frequency mode
table which appears to be read-only on i.MX7. So while we are at it,
simplify the code by adding set_cntfrq() implementation for ARMv7 and
copy the code we already using for i.MX8MQ.

Fixes: dece70752 ("clocksource: Enable architected timer support for CPU_V7")
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>