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/arch/arm/mach-zynq/Kconfig b/arch/arm/mach-zynq/Kconfig
index d28e5fc..5a1f7ca 100644
--- a/arch/arm/mach-zynq/Kconfig
+++ b/arch/arm/mach-zynq/Kconfig
@@ -12,6 +12,9 @@
 config SPL_LIBGENERIC_SUPPORT
 	default y
 
+config SPL_MMC_SUPPORT
+	default y if ZYNQ_SDHCI
+
 config SYS_BOARD
 	default "zynq"