rtc: amlogic-a4: Optimize global variables

Convert a global variable into a local one of aml_rtc_probe().

Signed-off-by: Xianwei Zhao <xianwei.zhao@amlogic.com>
Link: https://lore.kernel.org/r/20250722-rtc-regmap-v2-1-58bc17187a11@amlogic.com


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