[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>

arch/mips/txx9/generic/setup.c

@@ -328,6 +328,9 @@ void __init arch_init_irq(void)
 
 void __init plat_time_init(void)
 {
+#ifdef CONFIG_CPU_TX49XX
+	mips_hpt_frequency = txx9_cpu_clock / 2;
+#endif
 	txx9_board_vec->time_init();
 }