The mpu_clk, main_clk, and dbg_base_clk outputs from the main PLL go through a
pre-divider. Update socfpga.dtsi to represent those dividers for these
clocks.

Re-use the "div-reg" property that was used for the socfpga-gate-clock as this
is the same thing. Also update the documentation.
Signed-off-by: Dinh Nguyen <dinguyen@altera.com>