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/pico-dwarf-imx7d_defconfig b/configs/pico-dwarf-imx7d_defconfig
index 07b17f5..faa6a1e 100644
--- a/configs/pico-dwarf-imx7d_defconfig
+++ b/configs/pico-dwarf-imx7d_defconfig
@@ -65,6 +65,7 @@
 CONFIG_MII=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_IMX7=y
+CONFIG_MXC_UART=y
 CONFIG_USB=y
 CONFIG_DM_USB=y
 CONFIG_USB_EHCI_HCD=y