arm: mediatek: remove arch_misc_init

The watchdog of mediatek chips is enabled by bootrom before u-boot is
running. Previously we choose to enable the wdt driver only to disable the
watchdog hardware.

Now wdt service is enabled by default. The function arch_misc_init which is
only used to disable wdt is no longer needed.

Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>