watchdog: imx2_wdt: adds big endianness support.

This watchdog driver will be working on IMX2+, Vybrid, LS1, LS2+
platforms, and will be in different endianness mode in those SoCs:

    SoCs       WDT endian mode
------------------------------------
    IMX2+           LE
    Vybird          LE
    LS1             BE
    LS2             LE

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>