MIPS: ath79: Fix the ar724x clock calculation

According to the AR7242 datasheet section 2.8, AR724X CPUs use a 40MHz
input clock as the REF_CLK instead of 5MHz.

The correct CPU PLL calculation procedure is as follows:
CPU_PLL = (FB * REF_CLK) / REF_DIV / 2.

This patch is compatible with the current calculation procedure with
default FB and REF_DIV values.

Tested on AR7240, AR7241 and AR7242.
Signed-off-by: Weijie Gao <hackpascal@gmail.com>
Signed-off-by: Alban Bedel <albeu@free.fr> (Fixed the commit log message)
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12870/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/ath79/clock.c

@@ -26,7 +26,7 @@
 #include "common.h"
 
 #define AR71XX_BASE_FREQ	40000000
-#define AR724X_BASE_FREQ	5000000
+#define AR724X_BASE_FREQ	40000000
 #define AR913X_BASE_FREQ	5000000
 
 static struct clk *clks[3];
@@ -103,8 +103,8 @@ static void __init ar724x_clocks_init(void)
 	div = ((pll >> AR724X_PLL_FB_SHIFT) & AR724X_PLL_FB_MASK);
 	freq = div * ref_rate;
 
-	div = ((pll >> AR724X_PLL_REF_DIV_SHIFT) & AR724X_PLL_REF_DIV_MASK);
-	freq *= div;
+	div = ((pll >> AR724X_PLL_REF_DIV_SHIFT) & AR724X_PLL_REF_DIV_MASK) * 2;
+	freq /= div;
 
 	cpu_rate = freq;