m68k: mac: Improve clocksource driver commentary

qemu-system-m68k -M q800 has an old bug that causes the kernel to
occasionally complain about a soft lockup:

    watchdog: BUG: soft lockup - CPU#0 stuck for 5107s!

There isn't any actual lockup. The via1 clocksource produced a large
jump in jiffies, causing the watchdog to detect a stale timestamp.

The 32-bit clocksource counter runs at 783360 Hz and its period is
about 5482 seconds. Applying the "nanosecond" approximation used in
get_timestamp() in kernel/watchdog.c then yields the duration reported
in the log message above (always 5107 or 5108 in my tests):

0xffffffff / VIA_CLOCK_FREQ * 10**9 / 2**30 = 5106.209 seconds

It is notoriously difficult to correctly emulate a MOS6522 VIA chip. So
it seems wise to document the VIA clocksource driver better, especially
those hardware behaviours which the kernel relies upon.

Cc: Joshua Thompson <funaho@jurai.org>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/f7b4c02a1c8ed74ccceb5535d7e1e202deada8ce.1750739568.git.fthain@linux-m68k.org
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

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