Add enum for mips breakpoint kinds

This patch adds an enum mips_breakpoint_kind to avoid using magic
numbers as much as possible.

gdb:

2016-11-03  Yao Qi  <yao.qi@linaro.org>

	* mips-tdep.c (mips_breakpoint_kind): New enum.
	(mips_breakpoint_from_pc): Use it.
	(mips_remote_breakpoint_from_pc): Likewise.