[MIPS] TXx9: Fix mips_hpt_frequency initialization

The mips_hpt_frequency initialization code was lost in commit
94a4c329 (linux-mips.org) /
94a4c329 (kernel.org) "TXx9: Add 64-bit
support".

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