m68k: Rename CONFIG_WATCHDOG_TIMEOUT to CONFIG_WATCHDOG_TIMEOUT_MSECS

In practice, it is clear that the usage in m68k of
CONFIG_WATCHDOG_TIMEOUT is setting a value in milliseconds. Rename this
to the existing symbol and move to Kconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/M5235EVB_Flash32_defconfig b/configs/M5235EVB_Flash32_defconfig
index 1b7b156..c0087aa 100644
--- a/configs/M5235EVB_Flash32_defconfig
+++ b/configs/M5235EVB_Flash32_defconfig
@@ -50,3 +50,4 @@
 CONFIG_MCFFEC=y
 CONFIG_MII=y
 CONFIG_MCFUART=y
+CONFIG_WATCHDOG_TIMEOUT_MSECS=5000