[FR-V] Handle FR300

[FR-V] Handle FR300

Even though "set architecture" presents fr300 as option:

 (gdb) set architecture fr<TAB>
 fr300  fr400  fr450  fr500  fr550  frv

Actually selecting fr300 doesn't work:

 (gdb) set architecture fr300
 Architecture `fr300' not recognized.
 The target architecture is set automatically (currently i386)
 (gdb)

This just looks like an obvious oversight.  Looking around gcc and
binutils sources, FR300 is basically a FR500 specialized for DSP and
low power.

gdb/ChangeLog:
2016-03-09  Pedro Alves  <palves@redhat.com>

	* frv-tdep.c (frv_gdbarch_init): Handle bfd_mach_fr300.