Convert CONFIG_FSL_USDHC to Kconfig

This converts the following to Kconfig:
   CONFIG_FSL_USDHC

Signed-off-by: Adam Ford <aford173@gmail.com>
[trini: Add IMX8M, TARGET_S32V234EVB to FSL_USDHC list]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/mx6memcal_defconfig b/configs/mx6memcal_defconfig
index ebb48c3..d20e3db 100644
--- a/configs/mx6memcal_defconfig
+++ b/configs/mx6memcal_defconfig
@@ -36,6 +36,7 @@
 CONFIG_CMD_CACHE=y
 # CONFIG_NET is not set
 # CONFIG_MMC is not set
+CONFIG_FSL_USDHC=y
 CONFIG_USB=y
 CONFIG_USB_GADGET=y
 CONFIG_USB_GADGET_MANUFACTURER="FSL"