stm32mp2xx_ll_rcc: add support for I2C8 clock configuration

This commit adds support for retrieving the I2C8 device clock
configuration in the HAL RCC driver. This will be used by the Zephyr
clock driver to read the I2C8 clock frequency in the same way as I2C1-7.

(cherry picked from commit d46f8453c5ce28fb6aba7613de7ebc75bd8572c3)

Original-Signed-off-by: Tanguy Raufflet <tanguy.raufflet@savoirfairelinux.com>
GitOrigin-RevId: d46f8453c5ce28fb6aba7613de7ebc75bd8572c3
Cr-Build-Id: 8708647255185269425
Cr-Build-Url: https://cr-buildbucket.appspot.com/build/8708647255185269425
Copybot-Job-Name: zephyr-hal_stm32-copybot-downstream
Change-Id: I89a18a41279c5808b46a60dabd6bc7869b63bd78
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/hal_stm32/+/6777720
Commit-Queue: ChromeOS Copybot <copybot@chops-service-accounts.iam.gserviceaccount.com>
Bot-Commit: ChromeOS Copybot <copybot@chops-service-accounts.iam.gserviceaccount.com>
Tested-by: ChromeOS Copybot <copybot@chops-service-accounts.iam.gserviceaccount.com>