Arndale port console should use SerialDxe

The Samsung Arndale port has certain limitations in its serial console support.
Backspace does not work, the countdown timeout messages are repeated rather than having their remaining time updated, and ascii graphics (in for example the GRUB boot menu) look completely garbled.
Also, the port explicitly changes the default serial console geometry away from industry standard 80x25.

The underlying reason for this is that the port uses the EmbeddedPkg/SimlpeTextInOutSerial libarary rather than the EmbeddedPkg/SerialDxe one. An additional requirement for SerialDxe is to implement SerialPortExtLib for the platform.

I have attached a patch against uefi-next commit 89d9d37f.. that enables the Arndale port to build with SerialDxe with a completely stubbed-out SerialPortExtLib taken from the VEXpress port, to serve as a starting point. The patch also reverts the overriding of console geometry.

The resulting image boots, with pretty ascii support, but console input is broken.