watchdog: mpc8xx: Rename it mpc8xxx

mpc8xx, mpc83xx and mpc86xx have similar watchdog with almost same
memory registers.

Rename it mpc8xxx which is the generic name used for drivers supporting
several mpc families.

The driver will be made more generic in following patch.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index bee59c3..f20d58b 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -31,7 +31,7 @@
 	select CREATE_ARCH_SYMLINK
 	select BOARD_EARLY_INIT_F
 	imply CMD_REGINFO
-	imply WDT_MPC8xx
+	imply WDT_MPC8xxx
 
 endchoice