stm32mp2xx.h: fix the USE_HAL_DRIVER include

Modify the included HAL header in stm32mp2xx.h to the correct one.
"stm32mp2xx_hal_conf.h" -> "stm32mp2xx_hal.h"

"stm32mp2xx_hal.h" was never included in build time and caused
declaration errors when using the HAL driver.

Signed-off-by: Youssef Zini <youssef.zini@savoirfairelinux.com>