At the end of the boot process, the clock framework might disable
required main PLL's. So far, this was no issue since drivers
requested clocks, which are descended of the main PLL's (e.g.
pll1_pfd1, which provides the system clock).

To archive the full 500MHz system clock, DDR clock need to be a
descendant of PLL2 rather than PLL1 (DDRC_CLK_SEL set to 0). The
bootloader sets up the clocks accordingly before making use of
DDR at all. However, in Linux, there is no driver using PLL2,
which lead to PLL2 being disabled by the clock framework.

With this patch, we make sure that the main system clock and the
DDR clock are initially enabled and are kept enabled.
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>