MIPS: Move timer code to arch/mips/cpu/$(CPU)/

Current timer routines (arch/mips/lib/timer.c) are implemented assuming
that MIPS32 coprocessor (CP0) resources, Counter and Compare registers
in this case, are available.  But this doesn't always work.

We need to make sure that all MIPS-based systems don't necessarily use
CP0 counter/compare registers as time keeping resources.  And some MIPS
variant processors might come with different hardware specs with genuine
MIPS32 CP0 registers.

With this change, each $(CPU)/ directory can have its own timer code.

Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>