serial/arc-uart: switch to devicetree based probing

* DT binding for arc-uart
* With alll the bits in place we can now use DT probing.

Note that there's a bit of kludge right now because earlyprintk portion
of driver can't use the DT infrastrcuture to get resoures/plat_data.
This requires some infrastructre changes to of_flat_ framework

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Alan Cox <alan@linux.intel.com>
Cc: devicetree-discuss@lists.ozlabs.org
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Rob Landley <rob@landley.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/tty/serial/arc-uart.txt

@@ -0,0 +1,26 @@
+* Synopsys ARC UART : Non standard UART used in some of the ARC FPGA boards
+
+Required properties:
+- compatible		: "snps,arc-uart"
+- reg			: offset and length of the register set for the device.
+- interrupts		: device interrupt
+- clock-frequency	: the input clock frequency for the UART
+- baud			: baud rate for UART
+
+e.g.
+
+arcuart0: serial@c0fc1000 {
+	compatible = "snps,arc-uart";
+	reg = <0xc0fc1000 0x100>;
+	interrupts = <5>;
+	clock-frequency = <80000000>;
+	baud = <115200>;
+	status = "okay";
+};
+
+Note: Each port should have an alias correctly numbered in "aliases" node.
+
+e.g.
+aliases {
+	serial0 = &arcuart0;
+};