watchdog: imx2_wdt: use preferred BIT macro instead of open coded values

This is a nonfunctional change, declare register bit values with BIT()
helper macro.

The issues are reported by checkpatch:

  CHECK: Prefer using the BIT macro
  #40: FILE: drivers/watchdog/imx2_wdt.c:40:
  +#define IMX2_WDT_WCR_WDA	(1 << 5)	/* -> External Reset WDOG_B */

etc.

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>