[MIPS] lib_mips/time.c: Replace CP0 access functions with existing macros

We already have many pre-defined CP0 access macros in <asm/mipsregs.h>.
This patch replaces mips_{compare,count}_set and mips_count_get with
existing macros.

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>