fix(st): don't try to read boot partition on SD cards

fix(st): don't try to read boot partition on SD cards

When trying to boot from an SD card with STM32MP_EMMC_BOOT enabled,
booting fails with:

	ERROR:   Got unexpected value for active boot partition, 0
	ASSERT: plat/st/common/bl2_stm32_io_storage.c:285

because SD cards don't provide a boot partition. So only try reading
from such a partition when booting from eMMC.

Fixes: 214c8a8d08b2 ("feat(plat/st): add STM32MP_EMMC_BOOT option")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Change-Id: I354b737a3ae3ea577e83dfeb7096df22275d852d