Use definitions instead of magic values in order to improve readability.

Since the CLK field of the WDOG CS register is composed of two bits
to select the watchdog clock source, use a shift representation
instead of BIT().
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20191029174037.25381-5-festevam@gmail.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>