watchdog: imx2_wdt: Select the driver via ARCH_MXC

With device tree support in place, we should not use IMX_HAVE_PLATFORM_IMX2_WDT
as a dependency for selecting the imx2_wdt driver.

Use ARCH_MXC symbol instead, so that the driver can be even selected by a device-tree
only SoC, such as i.MX6.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>