pico: Convert to CONFIG_FSL_USDHC

The eSDHC config symbol has been converted to CONFIG_FSL_USDHC, so
adjust it accordingly in order to fix the build errors:

https://travis-ci.org/sbabic/u-boot-imx/jobs/584781580#L1287

Reported-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
diff --git a/configs/pico-dwarf-imx6ul_defconfig b/configs/pico-dwarf-imx6ul_defconfig
index 33bb70b..f1f6506 100644
--- a/configs/pico-dwarf-imx6ul_defconfig
+++ b/configs/pico-dwarf-imx6ul_defconfig
@@ -44,7 +44,7 @@
 CONFIG_DM_GPIO=y
 CONFIG_DM_I2C=y
 CONFIG_DM_MMC=y
-CONFIG_FSL_ESDHC=y
+CONFIG_FSL_USDHC=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y
 CONFIG_MII=y