Fix display of RL78 MOVW instructions that use the stack pointer.

Fix display of RL78 MOVW instructions that use the stack pointer.

	* rl78-decode.opc (rl78_decode_opcode): Add 's' operand to movw
	instructions that can support stack pointer operations.
	* rl78-decode.c: Regenerate.
	* rl78-dis.c: Fix display of stack pointer in MOVW based
	instructions.

	* testsuite/gas/rl78/sp-relative-movw.s: New test.
	* testsuite/gas/rl78/sp-relative-movw.d: Expected disassembly.
	* testsuite/gas/rl78/rl78.exp: Run the new test.