watchdog: i.MX: use IMX_HAVE_PLATFORM_IMX2_WDT to depend on

The i.MX architecture provides IMX_HAVE_PLATFORM_* macros to signal
that a selected SoC supports a certain hardware. Use them instead of
depending on ARCH_* directly.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: linux-watchdog@vger.kernel.org