[WATHDOG] delete unused driver mpc8xx_wdt.c

The watchdog driver mpc8xx_wdt.c was a device interface to
arch/ppc/syslib/m8xx_wdt.c for MPC8xx hardware. Now that ARCH=ppc is
gone, this driver is of no more use. For ARCH=powerpc, MPC8xx hardware
is supported by mpc8xxx_wdt.c.

Signed-off-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Acked-by: Vitaly Bordug <vitb@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>