The stm32mp157c RM0436 reference manual lists following sequence for
resetting the SDMMC:

1. Reset using the RCC.SDMMCxRST register bit
  -> We do this via reset_control_assert

2. Disable Vcc power to the card
  -> We don't do this, because the rail often has other 3v3 peripherals
     hanging off it

3. Set the SDMMC to power-cycle state
  -> We do this only when the MCI core sets the frequency to zero, not
     on init!

4. reenable Vcc power to the card after a >1ms
  -> We don't do this, because we don't disable Vcc

5. Wait >1ms for the power ramp period, then put card into power-off
  -> We do this in stm32_sdmmc2_pwron

6. Wait >1ms, then put card into power-on state
  -> We do this in stm32_sdmmc2_pwron

7. Wait 74 SDMMC_CK cycles before sending the first command to the card
  -> We don't have an explicit delay between mci_startup_{sd,mmc} and
     mci_set_blocklen, so a low max-speed may run afoul of the timing
     constraint

Fix 3 & 7. 3 is the main one and doing it fixes an issue of timeouts
occurring, when having barebox in a reboot loop or the SD-Card inside
a USB-SD-Mux.

Fixes: ce99d0c86b32ec ("mci: add support for stm32mp sd/mmc controller")
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>