MIPS: use common code from lib/time.c

The common code just needs the C0_COUNT as free running counter,
without the need of writing and checking C0_COMPARE.

The function get_tbclk() is still implemented here instead of changing
all places of CONFIG_SYS_MIPS_TIMER_FREQ to CONFIG_SYS_TIMER_RATE.

The change was tested on a MIPS32 system, but as the MIPS64 code
was/is the same, this should be no problem.

Signed-off-by: Thomas Langer <thomas.langer@lantiq.com>