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>