drivers: rtc: set 'rtc_driver_api' as 'static const'

This change marks each instance of the 'rtc_driver_api' as 'static const'.
The rationale is that 'rtc_driver_api' is used for declaring internal
module interfaces and is not intended to be modified at runtime.
By using 'static const', we ensure immutability, leading to usage of only
.rodata and a reduction in the .data area.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>