ARM: mach-imx: Kconfig: Remove IMX_HAVE_PLATFORM_IMX2_WDT from SOC_IMX53

SOC_IMX53 is a device-tree only platform, so we don't need
IMX_HAVE_PLATFORM_IMX2_WDT at all because this symbol only provides some code
to help register imx2-wdt platform devices.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>