MIPS: Oprofile: Rename cpu_type from godson2 to loongson2

Unify the naming method between kernel and the user-space oprofile tool.
Because loongson is used instead of godson in most of the places, we agreed
to use loongson instead, which will simplify future maintenance.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Acked-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>