watchdog: txx9wdt: Add missing clock (un)prepare calls for CCF

While the custom minimal TXx9 clock implementation doesn't need or use
clock (un)prepare calls (they are dummies if !CONFIG_HAVE_CLK_PREPARE),
they are mandatory when using the Common Clock Framework.

Hence add them, to prepare for the advent of CCF.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>