ARC: [axs101] support early 8250 uart

Earlycon calculates UART clock as "BASE_BAUD * 16". In case of ARC
"BASE_BAUD" is calculated dynamically in runtime, basically it is an
alias to arc_early_base_baud(), which in turn just does
"arc_base_baud/16".

8250 UART on AXS/SDP board uses 33.3MHz clock source which is set in
"arc_base_baud" with this change.

Additional compatibility string "snps,arc-sdp" is introduced as well
because there're different flavours of AXS boards but they all share the
same motherboard and so it's possible to re-use the same code for
motherbord even if CPU daughterboard changes.
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

Documentation/devicetree/bindings/arc/axs101.txt

@@ -4,4 +4,4 @@ Synopsys DesignWare ARC Software Development Platforms Device Tree Bindings
 SDP Main Board with an AXC001 CPU Card hoisting ARC700 core in silicon
 
 Required root node properties:
-    - compatible = "snps,axs101";
+    - compatible = "snps,axs101", "snps,arc-sdp";

arch/arc/boot/dts/axs101.dts

@@ -13,9 +13,9 @@
 /include/ "axs10x_mb.dtsi"
 
 / {
-	compatible = "snps,axs101";
+	compatible = "snps,axs101", "snps,arc-sdp";
 
 	chosen {
-		bootargs = "console=tty0 console=ttyS3,115200n8 consoleblank=0";
+		bootargs = "earlycon=uart8250,mmio32,0xe0022000,115200n8 console=tty0 console=ttyS3,115200n8 consoleblank=0";
 	};
 };

arch/arc/kernel/devtree.c

@@ -32,6 +32,8 @@ static void __init arc_set_early_base_baud(unsigned long dt_root)
 
 	if (of_flat_dt_is_compatible(dt_root, "abilis,arc-tb10x"))
 		arc_base_baud = core_clk/3;
+	else if (of_flat_dt_is_compatible(dt_root, "snps,arc-sdp"))
+		arc_base_baud = 33333333;	/* Fixed 33MHz clk */
 	else
 		arc_base_baud = core_clk;
 }