Convert CONFIG_SYS_MMC_MAX_BLK_COUNT to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MMC_MAX_BLK_COUNT

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/configs/socfpga_sockit_defconfig b/configs/socfpga_sockit_defconfig
index 0539d29..31cc03a 100644
--- a/configs/socfpga_sockit_defconfig
+++ b/configs/socfpga_sockit_defconfig
@@ -54,6 +54,7 @@
 CONFIG_DWAPB_GPIO=y
 CONFIG_DM_I2C=y
 CONFIG_SYS_I2C_DW=y
+CONFIG_SYS_MMC_MAX_BLK_COUNT=256
 CONFIG_MMC_DW=y
 CONFIG_MTD=y
 CONFIG_SPI_FLASH_MACRONIX=y