watchdog: iop_wdt only builds for mach-iop13xx

All three iop variants we support in Linux (iop32x, iop33x and
iop13xx) seem to have support for the watchdog hardware, but this
driver fails to build for the first two of these because it
uses the IOP13XX_WDTCR_IB_RESET macro that is only defined for
iop13xx.

This clarifies the dependency in Kconfig to avoid randconfig
build errors. It is unlikely that anyone will ever miss support
for this driver on the ancient iop3xx platforms, so we don't
need to bother trying to fix it properly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: linux-watchdog@vger.kernel.org