rtc: zynqmp: Fix optional clock name property

Clock description in DT binding introduced by commit f69060c1
("dt-bindings: rtc: zynqmp: Add clock information") is talking about "rtc"
clock name but driver is checking "rtc_clk" name instead.
Because clock is optional property likely in was never handled properly by
the driver.

Fixes: 07dcc6f9 ("rtc: zynqmp: Add calibration set and get support")
Signed-off-by: Michal Simek <michal.simek@amd.com>
Cc: stable@kernel.org
Reviewed-by: Peter Korsgaard <peter@korsgaard.com>
Link: https://lore.kernel.org/r/cd5f0c9d01ec1f5a240e37a7e0d85b8dacb3a869.1732723280.git.michal.simek@amd.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>