Merge tag 'arc-uart-updates-for-2018.07-rc1' of git://git.denx.de/u-boot-arc

Add support for DEBUG_UART on ARC devboards

This required us to do 2 things:
 1) Insert a call to debug_uart_init() in early boot code
 2) Convert serial_arc to Kconfig

Once both items above are done we just patched defconfigs.