Convert CONFIG_MXC_UART to Kconfig

This converts the following to Kconfig:
   CONFIG_MXC_UART

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Peng Fan <peng.fan@nxp.com>
diff --git a/configs/novena_defconfig b/configs/novena_defconfig
index 7bc3f28..dff60c1 100644
--- a/configs/novena_defconfig
+++ b/configs/novena_defconfig
@@ -59,6 +59,7 @@
 CONFIG_PCI=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_IMX6=y
+CONFIG_MXC_UART=y
 CONFIG_DM_THERMAL=y
 CONFIG_IMX_THERMAL=y
 CONFIG_USB=y