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