[MIPS] mips hpt cleanup: get rid of mips_hpt_init

Currently nobody outside time.c require mips_hpt_init().  Remove it
and call c0_hpt_timer_init() directly if R4k counter was used for
timer interrupt.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>