MIPS: csrc-sb1250: Extract hpt cycle acquisition from sb1250_hpt_read

This is to prepare for the upcoming read_sched_clock implementation, which
will also need to get cycles from the high precision timer.

Signed-off-by: Deng-Cheng Zhu <dengcheng.zhu@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: macro@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9486/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>