ARM i.MX6: Fix ldb_di clock selection

According to the recent i.MX6 Quad technical reference manual, mode 0x4 (100b)
of the CCM_CS2DCR register (address 0x020C402C) bits [11-9] and [14-12] select
the PLL3 clock, and not the PLL3 PFD1 540M clock. In our code, the PLL3 root
clock is named 'pll3_usb_otg', select this instead of the 540M clock.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>