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 <[email protected]> BUG=none BRANCH=none TEST=built and booted on Pit Change-Id: I9fa85194ae1a1fadab79695f059efdc2e2f1f75f Reviewed-on: https://gerrit.chromium.org/gerrit/65611 Reviewed-by: Ronald G. Minnich <[email protected]> Tested-by: David Hendricks <[email protected]> Commit-Queue: David Hendricks <[email protected]>