target-mips: allow microMIPS SWP and SDP to have RD equal to BASE

The microMIPS SWP and SDP instructions do not modify GPRs.  So their
behavior is well defined when RD equals BASE.  The MIPS Architecture
Verification Programs (AVPs) check that they work as expected.  This
is required for AVPs to pass.

Signed-off-by: Eric Johnson <ericj@mips.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
