rtc: k3: wait until the unlock field is not zero

After writing the magic words to the KICK0 and KICK1 registers, we must
wait for a 1 in the unlock field of the general control register to
signify when the rtc device is in an unlocked state.

Signed-off-by: Bryan Brattlof <bb@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220816173312.23243-1-bb@ti.com