openrisc: fix trap for debugger breakpoint signalling

I have been working on getting native Linux GDB support working for the
OpenRISC port.  The trap signal handler here was wrong in a few ways.
During trap handling address (from the EEAR register) is not set by the
CPU, so it is not correct to use here.  We want to use trap as a
break-point so use TRAP_BRKPT.  Adding 4 to the pc was incorrect and
causing GDB to think the breakpoint was not hit.

Fixing these allows GDB to work now.

Signed-off-by: Stafford Horne <shorne@gmail.com>