feat(st-sdmmc2): define FIFO size

Instead of using hard-coded values in stm32_sdmmc2_read() function,
use a defined SDMMC_FIFO_SIZE, which is 64 on STM32MP1.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I1ace0a28fbddae474379f0187371b9c360ceb7b3
1 file changed