spi: stm32_qspi: Add reset support

In some situation, QSPI controller is already configured by an early
boot stage, adding reset support will insure that QSPI controller is
started from a pristine state.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>