ENGR00279402-1 ARM: dts: imx6: add wdog reset source seclect in dts

ENGR00279402-1 ARM: dts: imx6: add wdog reset source seclect in dts

Some boards use another WDOG reset source to reboot system in ldo-bypass mode.
We need add the property in board dts file so that we can easily know the
WDOG reset source currently.

For Sabresd, WDOG1 for ldo-enable mode(WDOG event), WDOG2 for ldo-bypass mode
(reset external pmic to trigger POR event).
For sl-evk board, there is no WDOG pin connected with external pmic as Sabresd
, because mx6sl boot at 400Mhz. Then both ldo-enable and ldo-bypass mode use
the common WDOG1 as reset source.

Signed-off-by: Robin Gong <b38343@freescale.com>
(cherry picked from commit d960bbe78009d306f80957321622aaad0628ac4b)