mmc: fsl_esdhc_spl: Call mmc_init() before booting from SD card

mmc: fsl_esdhc_spl: Call mmc_init() before booting from SD card

If env is stored on SD card then U-Boot SPL automatically calls mmc_init()
before it is going to load proper U-Boot from SD card.

If env is not stored on SD card then U-Boot SPL fails to read proper U-Boot
from SD card due to missing mmc_init() call.

So add missing mmc_init() call into fsl_esdhc_spl's mmc_boot() function.
It fixes booting from SD card on P2020 boards without env support in SPL.

mmc_init() returns early if card was already initialized, so there is no
issue with calling this function more times.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>