watchdog: imx2_wdt: convert to use regmap API.

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

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

Other possible SoCs:
SoCs         CPU endian mode      WDT endian mode
------------------------------------------------
Soc1             BE                    BE
Soc2             BE                    LE

And also the watchdog's registers will be 32-bits for some versions,
and though it is 16-bits in IMX2+, Vybird and LS+.

Using the regmap APIs, could be more easy to support different
endianness and also more easy to support 32-bits version...

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>