sample/zbus: qemu_nios2 needs faster tick for zbus test with picolibc

Picolibc is enough faster than the minimal C library that the zbus
benchmark will likely complete in well under 10ms on qemu_nios2. As this
target cannot provide a clock at higher resolution than the system tick, we
need to increase that rate to get a non-zero runtime for this benchmark.

Add new nios2-specific config variables that change the
CONFIG_SYS_CLOCK_TICKS_PER_SEC value, leaving all other platforms using the
standard value. We cannot just increase it for every platform as
qemu_arc_hs6x fails with a 1kHz rate.

Signed-off-by: Keith Packard <keithp@keithp.com>