exynos5420: don't assume MPLL for i2c parent clock

This reads the clock select field for MUX_ACLK_66_SEL in the
CLK_SRC_TOP1 register in order to obtain the source clock rate
for I2C peripherals. Before we were always assuming that the source
was the MPLL.

Unfortunately not all fields in the CLK_SRC_TOPn registers are
enumerated the same with regard to clock select. So this is just
a one-off for now.

This is basically ported from https://gerrit.chromium.org/gerrit/#/c/62443.

Signed-off-by: David Hendricks <dhendrix@chromium.org>

Change-Id: I9fa85194ae1a1fadab79695f059efdc2e2f1f75f
Reviewed-on: https://gerrit.chromium.org/gerrit/65611
Reviewed-by: Ronald G. Minnich <rminnich@chromium.org>
Tested-by: David Hendricks <dhendrix@chromium.org>
Commit-Queue: David Hendricks <dhendrix@chromium.org>
Reviewed-on: http://review.coreboot.org/4468
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

src/cpu/samsung/exynos5420/clock.c

@@ -231,7 +231,23 @@ unsigned long clock_get_periph_rate(enum periph_id peripheral)
 	case PERIPH_ID_I2C8:
 	case PERIPH_ID_I2C9:
 	case PERIPH_ID_I2C10:
-		sclk = get_pll_clk(MPLL);
+		/*
+		 * I2C block parent clock selection is different from other
+		 * peripherals, so we handle it all here.
+		 * TODO: Add a helper function like with the peripheral clock
+		 * select fields?
+		 */
+		src = (readl(&clk->clk_src_top1) >> 8) & 0x3;
+		if (src == 0x0)
+			src = CPLL;
+		else if (src == 0x1)
+			src = DPLL;
+		else if (src == 0x2)
+			src = MPLL;
+		else
+			return -1;
+
+		sclk = get_pll_clk(src);
 		div = ((readl(&clk->clk_div_top1) >> 8) & 0x3f) + 1;
 		return sclk / div;
 	default: