soc: arm: stm32l5: enable ICACHE
soc: arm: stm32l5: enable ICACHE

Enable Instruction Cache
Warning: no flash driver yet available for STM32l5
But cache coherency management (cache invalidate)
will be rerquired when implementing flash driver.
ICAHE must be disabled for any flash write opeartion.

Signed-off-by: Alexandre Bourdiol <c23cc711c078d3e6bb50b3dc45a1b8842691e59a@st.com>

galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr