mmc: stm32_sdmmc2: implement host_power_cycle callback

For the correct power cycle sequence with stm32_sdmmc2, the write of the
power cycle value in PWRCTL field of SDMMC_POWER register is now done in
stm32_sdmmc2_host_power_cycle() and no more in stm32_sdmmc2_pwrcycle().

Signed-off-by: Yann Gautier <yann.gautier@st.com>