ARC: use fixed frequencies in arc_set_early_base_baud()

UARTs usually have fixed clock so we're switching to use of
constant values instead of something derived from core clock
frequency.

Among other things this will allow us to get rid of
arc_{get|set}_core_freq() and switch to generic clock
framework later on.
Acked-by: Christian Ruppert <christian.ruppert@alitech.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/devtree.c

@@ -28,14 +28,12 @@ unsigned int __init arc_early_base_baud(void)
 
 static void __init arc_set_early_base_baud(unsigned long dt_root)
 {
-	unsigned int core_clk = arc_get_core_freq();
-
 	if (of_flat_dt_is_compatible(dt_root, "abilis,arc-tb10x"))
-		arc_base_baud = core_clk/3;
+		arc_base_baud = 166666666;	/* Fixed 166.6MHz clk (TB10x) */
 	else if (of_flat_dt_is_compatible(dt_root, "snps,arc-sdp"))
 		arc_base_baud = 33333333;	/* Fixed 33MHz clk (AXS10x) */
 	else
-		arc_base_baud = core_clk;
+		arc_base_baud = 50000000;	/* Fixed default 50MHz */
 }
 #else
 #define arc_set_early_base_baud(dt_root)