imx: Remove boolean parameter from wdog powerdown function

imx_set_wdog_powerdown() is always used to disable the power down
enable bit, so remove the boolean parameter of the function.

It is also a bit strange to write a boolean value into registers,
so this new version makes explicit that we are writing 0.

While at it, rename it to imx_wdog_disable_powerdown().

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>