wdt: dw: Switch to if(CONFIG()) instead of using #if

This is preferred over #if because the compiler can check syntax even if
the feature is disabled. This cannot be used for CONFIG_CLK because
CONFIG_DW_WDT_CLOCK_KHZ is not defined on all platforms.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>