target-mips: add user-mode FR switch support for MIPS32r5

Description of UFR feature:

Required in MIPS32r5 if floating point is implemented and user-mode FR
switching is supported. The UFR register allows user-mode to clear StatusFR
by executing a CTC1 to UFR with GPR[0] as input, and read StatusFR by
executing a CFC1 to UFR.

helper_ctc1 has been extended with an additional parameter rt to check
requirements for UFR feature.
Definition of mips32r5-generic has been modified to include support for UFR.

Signed-off-by: Petar Jovanovic <petar.jovanovic@imgtec.com>
Reviewed-by: Eric Johnson <eric.johnson@imgtec.com>
