watchdog: rti_wdt: Use managed APIs to handle runtime PM

commit ce8c01c08eb5571953bd800908a25692405f27ea from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git



Switch to devm_pm_runtime_enable() to simplify error handling in driver
probe.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>