target/mips: Fix cycle counter timing calculations

target/mips: Fix cycle counter timing calculations

The cp0_count_ns value is calculated from the CP0_COUNT_RATE_DEFAULT
constant in target/mips/cpu.c.  The cycle counter resolution is defined
per-CPU in target/mips/cpu-defs.c.inc; use this value for calculating
cp0_count_ns.  Fixings timing problems on guest OSs for the 20Kc CPU
which has a CCRes of 1.

Signed-off-by: Simon Burge <simonb@NetBSD.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20211213135125.18378-1-simonb@NetBSD.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>