watchdog: Remove MV64x60 watchdog driver

Commit 92c8c16f ("powerpc/embedded6xx: Remove C2K board support")
removed the last selector of CONFIG_MV64X60.

Therefore CONFIG_MV64X60_WDT cannot be selected anymore and
can be removed.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/9c2952bcfaec3b1789909eaa36bbce2afbfab7ab.1616085654.git.christophe.leroy@csgroup.eu


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>