soc: stm32: Fix backup_sram clock enable

This driver missed #32228 PR that converted STM32 drivers
to use DEVICE_DT_GET for clock activation.
Due to the renaming of STM32_CLOCK_CONTROL_NAME to
STM32_CLOCK_CONTROL_NODE, driver could not compile anymore

Fix this.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>