m68k: implement movep instruction

This patch implements movep instruction. It moves data between a data register
and alternate bytes within the address space starting at the location
specified and incrementing by two.

It was designed for the original 68000 and used in firmwares for
interfacing the 8-bit peripherals through the 16-bit data bus.
Without this patch opcode for this instruction is recognized as some bitop.

Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Signed-off-by: Mihail Abakumov <mikhail.abakumov@ispras.ru>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20180206124431.31433.91946.stgit@pasha-VirtualBox>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>