mach-stm32: Enable SPL_RESET_SUPPORT flag

Since commit 0e373c0a ("spl: add SPL_RESET_SUPPORT"),
reset is supported in SPL, enable this flag for STM32F SoCs family.

This allows to remove a specific case in RCC mfd driver.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>