target-openrisc: bugfix for dec_sys to decode instructions correctly

target-openrisc: bugfix for dec_sys to decode instructions correctly

Fixed the decoding of "system" instructions (starting with 0x2)
in dec_sys() in translate.c.  In particular, the l.trap instruction
is now correctly decoded, which enables for singlestepping and
breakpoints to be set in GDB.

Signed-off-by: David R. Morrison <dmorrison@invlim.com>
Acked-by: Jia Liu <proljc@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>