mips: Autodetect MIPS DSP rev1 and rev2 instructionset

Not every MIPS SoC has the DSP extensions, auto-detect them by using
builtin GCC functions. Check for the DSP macros and add the result
for rev1 and rev2 to the cpufeatures.

Change-Id: I3d6c950f170f102514c43b349f9a23ee796d801a
Reviewed-by: Girish Ramakrishnan <girish.1.ramakrishnan@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>