rtc: ds3232: avoid unused-const-variable warning

rtc: ds3232: avoid unused-const-variable warning

Fix the following warning when CONFIG_I2C is not set but CONFIG_SPI_MASTER
is set.

warning: 'ds3232_pm_ops' defined but not used [-Wunused-const-variable=]

'ds3232_pm_ops' is only used by rtc-ds3232 i2c driver, so move the device
PM callbacks inside #if IS_ENABLED(CONFIG_I2C).

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311172325.33tTniaJ-lkp@intel.com/
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Link: https://lore.kernel.org/r/20231118092200.829808-1-akinobu.mita@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>