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.

Signed-off-by: Tanguy Raufflet <tanguy.raufflet@savoirfairelinux.com>