Convert CONFIG_SPL_MMC_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/CHIP_defconfig b/configs/CHIP_defconfig
index a31aea0..296a0b5 100644
--- a/configs/CHIP_defconfig
+++ b/configs/CHIP_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_SUNXI=y
 CONFIG_SPL_I2C_SUPPORT=y
+# CONFIG_SPL_MMC_SUPPORT is not set
 CONFIG_MACH_SUN5I=y
 CONFIG_DRAM_TIMINGS_DDR3_800E_1066G_1333J=y
 # CONFIG_MMC is not set