Input: snvs_pwrkey - add clk handling

Input: snvs_pwrkey - add clk handling

On i.MX7S and i.MX8M* (but not i.MX6*) the pwrkey device has an
associated clock. Accessing the registers requires that this clock is
enabled. Binding the driver on at least i.MX7S and i.MX8MP while not
having the clock enabled results in a complete hang of the machine.
(This usually only happens if snvs_pwrkey is built as a module and the
rtc-snvs driver isn't already bound because at bootup the required clk
is on and only gets disabled when the clk framework disables unused clks
late during boot.)

This completes the fix in commit 8df6dc555325 ("ARM: dts: imx7s: add
snvs clock to pwrkey").

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20211013062848.2667192-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>